Your initial thought was brought up in the article - that we can only take this result at surface value because we have no way of judging the thought behind it.

Additionally, what you are talking about has been a topic of discussion in the behavioral sciences for the past decade+ at least since I've been working in it, probably longer. That the older behavioral papers discussing which animals were intelligent and basing it on things like the mirror test were only taking into consideration OUR senses and what we consider the best uses of them and how we judge cognition in our own species, but other species rely so heavily on other senses and interpret the world so differently that we need to judge based on THEIR level of understanding the world and how well their systems fit their needs.

So, while the mirror tests isn't used in the same way it eats in the old days (to test their self awareness to compare to each other and humans), it's used to see what we can learn about their senses and how they use them.