I don’t think it’s an attempt at humor. When I ask what’s in my Starbucks drink, all parties assume I want to know about the caffeine payload. All I care about is 400mg caffeine served in some quantity of something tasty. Plus if I care about calories, then it’s useful to know it’s 8 ounces.
The funny thing isn’t what the barista said, but the tacit acknowledgement that it’s basically a drug deal.
Neither do I. I don’t think the guy was trying to be funny.
And: I wasn’t at Starbucks.
No, the funny thing is that after giving no real information about what a flat white is, he ends with an off-hand, "“We argue with each other about what kind of milk to use.”
It’s unexpected and random and completely irrelevant to the question. That’s funny.
Okay, Frylock, now they’ve frustrated me into being firmly on your side. 
Yaaaaay! 
Out of curiosity, what is your definition of a flat white?
Now those comments all in the same thread by the same poster are mildly humorous…
I don’t drink basic coffee, not even mentioning all the permutations Starbucks offers, so I am certainly not your target audience, but it has the feeling of a Seinfeld TV Joke, IMHO.
And I never thought Seinfeld was funny, either.
What in the holy moly are you talking about?
One does not assume that the only way for someone to say something funny is to intentionally make a joke, correct?
How about instead of “funny” you read it as “frustratingly inadequate”? Would that work for you?
Du you have a big beard and wear lumberjack shirts?
I wouldn’t find it funny- maybe odd or irritating, but it doesn’t read funny per se to me. I’d be more likely to roll my eyes a bit at the unhelpfulness of the answer you got. If I saw that on someone’s feed (without a LOL or smilie face) I would have thought it was a call for clarification, that you still needed the info about what goes into a flat white, or someone to share in the roll-eyes moment.
I get how it would be a funny moment to experience, when approached with a particular mindset. I don’t think the post repeating the conversation is funny. The barista’s response is not a complete definition of a flat white, but most people don’t express themselves with academic precision, so the response does not indicate that he doesn’t know how to make a flat white. If you asked a builder “How do you build a house?” he probably would just spit out a couple of vague statements about foundations, frames, and whatnot. If you then went and told all your friends about the crazy builder who didn’t even know how to build a house, that would not be funny. That would be you being an overly literal drip.
You guys are weird.
I don’t know what a “flat white” is exactly, so I don’t know what sort of information you might be looking for. I know what nachos are so your example there isn’t quite the same. It’d be like if I relayed a tale to you:
“I went into the widget store and asked the guy behind the counter about the capacity of their Whitton fobs. He said, ‘About .75 liters.’”
See, that’s hilarious because I was actually asking about their electrical storage capacity, which everyone familiar with Whitton fobs would immediately understand. But this guy, he was so clueless!
That’s maybe a bit of an extreme example, but I don’t know from your OP if you’re a flat white aficionado looking for some detailed information about flat whites that might not make any sense to me otherwise, or if you’re a layman just wondering what the heck the drink is all about. Since I don’t immediately understand what’s funny about it, I’m inclined to assume the former; that is, I must not be in on the joke so I’d move on.
Yeah it does look like I mis-estimated what constitutes common knowledge about coffee. It’s not so much that you need to know what a flat white is specifically, as just that you need to know that for coffee drinks (cappucinos, lattes, machiattos, you know, all those starbucks words except not frappucino ;)) the type of milk product used, and how the milk is prepared, is basically the definition of the drink. (Coffee-to-milk ratios are also important, but even there you could change the ratio without necessarily making it an entirely different drink type.)
That might be an attempt at humor, but I would regard it as annoying irony. Like “ok, you’re ironically hip, but this isn’t Facebook, tell me what you’re putting in my goddamn drink.”
I found it funny.
The initial response seemed totally inadequate to answer the question…then after the pause, the reveal that the ‘experts’ themselves couldn’t agree on how to accomplish the task. To me, that’s funny. Not ‘Fish Slapping Dance funny’, but enough to generate a slight grin at least.
The fact that a relevant piece of information may have been completely omitted (regarding the manner in which the milk is poured) was completely lost on me because I didn’t know exactly what a ‘flat white’ was before opening this thread, but there was still humor in the statement without that knowledge.
It’s also interesting to me that in this thread a lot of people are assuming that when I say something someone is said is funny, I must mean that they were trying to be funny. (Even though, btw, in the OP I explicitly use the word “unintentionally.”)
I have not previously realized that this was a fact about many people’s concept of funny. I will have to keep that in mind in the future.
This is exactly what I expected the response to be, including the aspect that it doesn’t really matter what a flat white is.
Glad you got it. 
“How do you guys do a Latte?”
“Two shots in 12 oz glass.”
“Okay, got it.”
because everyone knows what a Latte is.
So, how is that different than:
At least for me, I got what what you were saying. The barista (unintentionally) responded to you in a way you found funny. I think that was pretty clear in your post.
I just didn’t think it was funny. I thought it was odd or deserving or an eye-roll, but not funny. I’m sympathetic because I also find humor in things other people don’t always find humorous. But there’s no objective measure here. Just because not everyone found it funny doesn’t mean everyone doesn’t get your point. Or that it wasn’t legitimately funny to you.