Mirror tests we’re now realizing are also very biased towards human perception and cognition, at least as they’ve historically been conducted.

A dog won’t pass a visual mirror test for example, but they have zero issue with a scent based one.

Other times it may come down to more complex personal factors we’re not yet fully able to understand. In the one test I was able to find involving Asian Elephants they tested 3 different elephants all from the same group/enclosure at the Bronx zoo. Only one of the 3 that were involved passed the test, but she passed it without leaving any room for doubt.

The fish in the article, cleaner wrasse, have specifically evolved to recognize colors or shapes that are out of place as that’s how they identify the parasites they consume. Logically it makes sense they’d be better adapted to pass a visual mirror test than most animals.

If a small and otherwise unremarkable fish is able to pass the mirror test when conducted in a manner that coincides with how it has evolved to perceive the world then I don’t have any trouble believing it’s entirely possible many animals are capable of passing it. It’s likely more a matter of how it’s conducted, with the need for dedicated tests factoring in the animal’s capabilities being the critical factor. If you think about it, if you were abducted by aliens and they had you try to identify yourself in a mirror that reflected infrared radiation, the spectrum they saw and perceived in, then you would fail too.