Actually, I do find credible that "lost persons" would die easily in such a world.
If they are mostly our average modern day person from a first-world country, they would probably have a hard time adapting to a dangerous environment like a medieval-fantasy world.
Add the arrogance of getting all new supernatural skills that are not outright OP, and I can imagine them rushing to "adventure" and getting killed quickly by random monsters. It might not even be a problem with skills, but with the state of mind. (Seriously, how many of you readers would keep a calm mind when facing bloodthirsty monsters?)
If they are soldiers or hunters or something similar, they might have a chance. But the random office worker doesn't stand much of a chance. (Obviously though, the MC will beat the odds.)
I bet the trainer here is more concerned with keeping the MC alive than looking down on him. And he doesn't yet know of his fight against a horde of goblins, so his bias seems justified.
At least, that's how it seems to me.