Interesting. I wonder how accurate it is. I live in the NYC area and know exactly two people with that accent. Three if you count my late grandmother.
My home town is listed as a New York accent, but no one spoke that way. It’s near to the Bonac area, but on the North Fork, so it’s not that, and no other dialect seems to match.
Based on my limited experience, it’s only accurate in the broadest strokes. I mean, people in say… Houston are probably more similar in accent to Mobile (both cities are in the Gulf Southern region) than to Kansas City (North Midland), but I think they sound closer still to people from Dallas (South Midland). even though that map shows Houston and Dallas being in two entirely different regions.
Bismarks? Jelly donuts are Bismarks in the upper Midwest and Rockies? Ok.
I wonder if anyone actually read the text below the map? The title of the page is “Dialect Map of American English” The first paragraph reads
IOW, the map is about dialects, not accents. So far we’re 3-1/2 for for 4 in talking about accents and how people in various regions sound. Dialect is mostly word choice, not so much about sound.
If they can’t spell Bismarck correctly, I wouldn’t trust them.
That’s how you can tell it’s a good bakery…
I’m an Inland Northern. It says that we pronounce Marry, Merry and Mary the same. How else would you pronounce them? 
I’m from the “North Midland” area, but spent several years of my childhood in a city on the border between there and “Upper Midwestern.”
Imagine how surprised I was, almost fifteen years after moving away from that city, that I’ve picked up the accent (not necessarily the dialect) of the Upper Midwest. I totally wear saaaahks instead of socks, and see the color bree-awn instead of brown.
I remember the fun of being on a camping trip with a Hoi Toider as a kid. “Oh no, my marshmallow fell in the foyer.”
They have separate pronunciations in my dialect of English.
The Dialect Survey Maps (link at the bottom of the page) is fun to read. It is a series of over a hundred examples of dialect, such as “What do you call the insect that flies around in the summer and has a rear section that glows in the dark?” with the results tagged on the map.
You can also drill down to a particular state to see the percentages of how each word is used or pronounced in that state based on their survey.
When I moved to NYC, I had to learn the 3 different pronunciations. Then, when I relocated back here, I had to re-learn to pronounce them all the same. And then there’s the soda/pop war.
I suppose you have never been to New Jersey?
Do you also pronounce the following the same?
fairy ferry
scary skerry
hairy Harry
berry Barry
carry Kerry
Try the following sentence. Are the underlined words all rhyming words and/or homophones, or do they fit into several categories of words that are either rhyming or homophonic?
Scary Terry picked a berry for merry Harry and took the ferry to the skerry, where he met Larry, Mary, and Jerry. Mary pulled a sword and asked Terry to parry. In the end they were all very merry and Harry asked Mary to marry him.
Some of the “Coastal Southern” terms listed I have never heard before - “They call doughnuts a cookie”? Never, ever have I heard a doughnut called a cookie - anywhere. “Sunday Child”, “kernal” and “dope” in their listed senses are archaic at best, I may have seen them used as such in literature. “Catty corner” and “fuss box” are still fairly common used. And I had no idea that “savannah” was dialectic. I thought it meant “grassland” pretty much everywhere.
ETA: Ref:
I don’t know what a “skerry” is, but I pronounce all the other words *exactly *the same. And the “skerry” I pronounced exactly like “scary” (AKA frightening).
Raised in So Cal, lived in NYC & the Midwest, and now Miami.
I’ve read about the differing pronounciations many times here. I still cannot conceive of it. I also don’t think I’ve every heard anyone use the differing pronuncations. This despite working on a daily basis with people who grew up, and presently live, all over the US, from corner to corner and everywhere in between.
I pronounce all of those words the same as its counterpart.
Take the ar sound from harbour and that’s what I use to pronounce Harry.
Take the er sound from Herr and that’s what I use to pronounce hairy.
I am tempted to do a quick recording to illustrate how I pronounce them differently.
Ok I recorded robert_columbia’s pronunciation test yokey. SoundCloud - Hear the world’s sounds
Of course I’m not American so it’d probably be more interesting to get a recording of an American for whom these differ.
I am from New York and we also pronounce each of those words differently. No one here would confuse one for the other.
Of course, rarely does anyone here have the stereotypical accent that the link in the OP referred to as the NYC accent, ie “Toity Toid street.” Maybe in a '30s gangster movie…