::I guess that's where the misunderstanding is.  You absolutely shoot enemies in bullet hell games too.  The things that differentiate a bullet hell from a regular shoot 'em up are basically two things: 1) the screen literally fills with hundreds/thousands of slow-moving bullets, and 2) the hitbox of your ship is much smaller than the ship itself.  As neither of those apply to Undertale, I don't see how it's anything but deceptive to use that term.