I was really excited to see Tsukinoki question the medium like that! I took a few classes with an art history professor that was infamous for asking that in every single critique he did. The studio artists hated it, because from a practical standpoint the student's answer was usually along the lines of "well, I made a bronze sculpture because I'm a sculptor and bronze is accessible and familiar."
But I do think it's a really interesting question, and it made my professor happy when people tried to answer it. In the case of oil painting, it can even be as simple as some of the things that Ooba went over earlier in the series. "The portrait is a window into an implied, larger image that continues off of the borders of the easel." It's a question that's actually easier to answer with simpler and more traditional mediums, I think, because if you go for some elaborate installation, you need to justify everything in and about the room.
Yaguchi got confused and overthought it, and went off in the opposite direction he needed to, I think! I hope he realizes soon that the fundamentals that Ooba taught him are universal, and not something that he needs to shun and move away from. The professor I actually dislike is the one who slammed 'exam art' and caused this self-doubt!