They do look like side buttons but then it’s still unusual that be wouldn’t have his hand on them. I think most digital artists appreciate the ability to quickly draw and erase lines multiple times to get them just right and to do that he’d have reach up there every time. Unless he

a) is a genius who does not undo
2) has developed blind muscle memory for Garfield as suggested by another commenter or
• is simply posing for the camera. Anyone’s guess.

Also, I personally ignore the side buttons because they’re annoying to reach.