This is the first I'm hearing about Escher not being a "real" artist. I am completely willing to believe that this is because I am a soulless math nerd who only talks about art with other soulless math nerds and engineers and we all love him because he made art about soulless math nerd things, but it's still news to me.
I was in a humanities class. There was an Escher exhibit a hundred miles away. I was not allowed to count my visit to the Escher exhibit as a necessary extracurricular "art experience" because Escher was deemed to not be an artist. To your point, the first Escher book I ever saw was owned by one of my math teachers. Four years previously we all piled in the school bus to see the Armand Hammer Collection, despite the fact that it's mostly art stolen from Jews by the Soviets.