Parmandur
Book-Friend, he/him
There is nothing to suggest in the books, and plenty to indicate otherwise, that a stat block is not an all-encompassing simulation. Thst has never been the case in 5E.But they're all they give us to work with. You have to assume that's what they consider a monster to be, because they don't give us anything else.
Until Level Up's Monster Menagerie came out, my favorite monster book for D&D by far was the 2e Monstrous Manual. That's a book that tried to give a full picture of it's subjects. It wasn't perfect by any means, but it was designed for what I wanted.