TL;DR It's fantasy.

You even decided to try and apply apply real world logic to this, however you're falling a bit short. Is the problem because of supposed musculature being in the way? That's generally not a problem since muscles can generally be relaxed and contracted to a pretty good degree which would decrease any such problems quite drastically. Additionally, how do you know the base is not just composed of fat and connective tissue to form some alternative support structure? Maybe the skeletal structure is vastly different than what's "normal" making this perfectly functional.

There's just so many things that could make this viable that it's not even worth to really argue about, because no-one can be either right or wrong, except for the artist/commissioner, their word is the only thing that has any weight, and then only in this specific case.

But since this isn't real making any such assumptions is quite silly. The anus might not be misplaced at all, it might be placed exactly where it's supposed to go, everyone got their idea of what is "normal" but that doesn't mean everyone else is going to hold up to that ideal.

Also since this is a rant, a small amount of salt is recommended when reading and before replying.