If that’s what makes it hard then it’s a number difficulty

I disagree. Giving both the player and bosses 1HP means there are no numbers because one hit kills you (and them). So the only way to adjust difficulty is through mechanics, as in boss movesets, constraints on controls, etc. It’s quite literally the opposite of a “numbers” game.

The opposite would be a typical JRPG. Bosses are more difficult if you’re under-leveled, easier if you’re over-leveled. Getting better stats, gear, etc makes the game easier because you have better numbers, so grinding is encouraged if you’re having trouble.

In short:

• “numbers” - adjust difficulty by adjusting access to better numbers (XP, gear, abilities, etc); players improve by grinding XP, gold, etc
• "mechanics" - adjust difficulty by adjusting mechanics (movesets, speed, variety of enemies on screen, etc); players improve by practicing