Thanks for playing,
The longer you survive, the more is explained about what the typing mechanic impacts. For the "big guys" you could either been seeing a hallucination that can't take damage, but also can't hurt you, or you just needed to shoot them more. Some of the enemies take more hits. You are right that typing words does slow down the enemies, but I would say "if you don't type, the enemies will speed up" is a better description.