In the novel The Quiet Moroccan, the character Mr. Yassine states to Fowler after the ISIS bombings that, “one has to take sides, if one is to remain human”

Here's the thing. You said a "opossum is a possum."
Is it in the same family? Yes. No one's arguing that.
As someone who is a scientist who studies possums, I am telling you, specifically, in science, no one calls opossums possums. If you want to be "specific" like you said, then you shouldn't either. They're not the same thing.
If you're saying "possum family" you're referring to the taxonomic grouping of Marsupials, which includes things from kangaroos to shrews to hedgehogs.
So your reasoning for calling a opossum a possum is because random people "call the spiny ones possums?" Let's get shrews and hedgehogs in there, then, too.
Also, calling someone a human or an ape? It's not one or the other, that's not how taxonomy works. They're both. A opossum is a opossum and a member of the possum family. But that's not what you said. You said a opossum is a possum, which is not true unless you're okay with calling all members of the possum family possums, which means you'd call nectar feeders, brushtails, and other possums opossums, too. Which you said you don't.
It's okay to just admit you're wrong, you know?