What's wrong with eating fungus?
I wouldn't blame him for an initial negative reaction. Eating unknown mushrooms is one of the easiest ways to get a deadly dose of poison on Earth. The dwarf recommended it, so it's potentially safe, but at the same time, maybe fantasy world dwarves can eat some things humans can't.
A technique against people would be far more advanced than one against monsters, since people are generally much more skilled. It's not so much the technique that's incompatible, but him who's unable to adapt to something much more straightforward.
A big part of martial arts would be to read and predict your opponent's moves. Reading a human opponent's moves is vastly different from reading a monster's moves (unless it's a humanoid, intelligent monster). Furthermore, all of your trained reactions could be worthless if the monster opponent moves in ways utterly unlike a human would move. On Earth, simply being left-handed was a big plus. For the majority, that is, right-handed, most opponents would also be right-handed. However, for a left-handed person, most opponents would still be right-handed. That's just statistics. So, most of the time a left-handed person would be fighting against those he's the most familiar with, whereas a right-handed encountering a left-handed opponent would face a bit bigger challenge, statistically. Change it to an animal/monster, and it would be even more out of ordinary.