Which naturally implies that a diet that’s free of vitamin C is safe for your pet (assuming it’s not a guinea pig, monkey, bat, or bony fish), but deadly for you.
That’s the same as California Buckeye and the European honeybee. Beekeepers absolutely hate the tree because it’s lethal to the imported honeybees. The coevolved native bumblebees and checkerspot butterflies which pollinate the tree aren’t bothered by it. Tough, beekeepers, it’s an interesting little native tree - note about the seeds - “This seed is the largest known of any temperate (non-tropical) plant species.”. About twice the size of the seed from the related horse chestnut. If kids still played “conkers”, it would be an impressive competitor. And anyway, it seems that the honeybees stay away from it if they can find anything else.
A little diarrhea doesn’t sound too bad if the alternative is dying. That should be a mythbusters-style “Plausible,” as in not BS, but the myth still doesn’t match reality.
Vitamin C is water soluble so any excess comes out in your pee. Vitamin A can be deadly if consumed in excess as it’s fat soluble and could build up (see post #5).
Well, diarrhea will dehydrate you.
Most cactuses are not good for this, including most species of the barrel cactus. The fishhook barrel cactus is the only one not toxic but contains oxalic acid and is likely to cause diarrhea if ingested on an empty stomach.
However, if you make a solar still, cactus pulp will help. BUT, it has been shown that most of the time you lose more water making such a still than you get back.
I’ve heard this often. Isn’t the point of these that they’re reusable though?
I know a guy who did it fora year. He ate nothing but raw meat. He had some nutty theory about how since vegetables are out to kill you (kinda true) you should never eat them (really not true. We’ve been successfully eating them for many generations.)
Your link supports the claim.
Okay, it’s not just “humans and guinea pigs”, it’s also some primates that are closely related to us, and bats, and a few non-mammals. But that’s pretty close.
It shows thousands of species (there are lots of species of bats and bony fish) can’t produce Vitamin C, but “thousands” is pretty close to “two”, right?
Well, maybe, if you have the right stuff to make one.
Sure, a one built with wood, plastic etc, carpentry can produce for a long time.
But a thin plastic sheet wont last long.
They do make a device for lifeboats, and those work OK.
Well, I don’t really think of fish when I’m counting “animals that do…”, and bats aren’t all that common, and are related to the primates. So, guinea pigs and stuff closely related to us.
{shrug}
From Wikipedia: “The second largest order of mammals, bats comprise about 20% of all classified mammal species worldwide, with over 1,200 species.”
While that’s just species count, not number of individuals there are far more than humans, guinea pigs, etc.
Being nocturnal cuts down on people seeing them but don’t assume they are uncommon at all.
Alas, due to white nose syndrome, they are incredibly rare where I live. 
I used to enjoy watching the bats at night. I wouldn’t be surprised if there are more guinea pigs in my county than bats. There are assuredly more humans in my county than there are bats. Twenty years ago, there were certainly more bats than guinea pigs, of course.
In general, humans and our livestock are a very large fraction of all mammals by biomass:
https://www.pnas.org/content/115/25/6506
But fine, humans, guinea pigs, and some animals that are closely related to humans that we don’t see very often are the only mammals who can’t manufacture their own vitamin C. We obviously interpret the original claim differently. I interpret it as “we’re weird for not making vitamin C”, which I think is true. I guess you interpret it as false because it includes a number (2) that isn’t accurate?
Oh – back to the “safe for pets, deadly for humans”
Most of the things that are “safe for humans and deadly for pets” are sort of exaggerated. That is, most of them are things that are more likely to kill your dog than to kill your child, not things that are completely safe vs. deadly poison. And that’s partly because we may be less susceptible to some toxin, but it’s largely because dogs wolf things down, whereas people tend to not eat things that “taste bad”, nor to eat vast quantities of anything. And most ordinary plant toxins are bitter and taste bad to people.
(and of course, there isn’t an enormous trade in dog food, with a premium placed on exotic and exciting options, among which we’d no doubt find things that we are more sensitive to than they are.)