>>23316143
I've been looking this up and the most I could find is that someone else, Ichiro Kojima, is credited for the designs, and he's a known artist who's worked on other games.
But yokoyama did work on the paint schemes/patterns, and also has a credit in the game for this. Any time he did art for other games he's properly credited though.

Maybe he supervised. Maybe this kojima guy was a bit of an understudy or something. I dunno.