It is absolutely an abandoned pet. They cannot survive outside the tropics. Hell, they can't even survive outside the 2 lakes in Mexico City that they're hyperadapted to
There are less than 1,000 of them in the wild. Trust me if it was possible to establish a population somewhere else outside of captivity, scientists and conservationists would already be on it
Examples in the wild are - bar the possibility of an albino example - all dark skinned. The pink/light skinned ones are the results of mutations and ultimately selective breeding in the pet population.
Not an axolotl, but mud puppies live as far north as Canada and have external gills: https://en.wikipedia.org/wiki/Common_mudpuppy
Olm in the Balkans: https://en.wikipedia.org/wiki/Olm picture: https://commons.wikimedia.org/wiki/File:P_anguinus-head1.jpg
So is it likely this one merely escaped? I find it hard to believe someone who would own one of these would not be an enthusiast, and that enthusiasts wouldn't find another owner for a critically endangered species rather than merely drop it under a local bridge.
> It is absolutely an abandoned pet.
That. Or the family fabricated the story for online fame.
Not saying that i have any evidence either way. Fundamentaly it is an unverifiable feel-good story with great online “viral” potential. It might be a very lucky axolotl who got abandoned, found and re-captured in the short window it could survive in the wild. It can also be a viral content strategy capturing eyeballs. In my, admitedly very jaded, guestimate I would give the two options about equal chances.