I can't speak to the applicability of the term as it applies to this particular game, but I've heard the term used for several years now. In my recollection, it started to come into vogue when Vampir Survivors blew up, but I wouldn't swear on that. The concept seems to be that, in a bullet hell game, the screen is filled with projectiles that you are attempting to avoid for fear of death. In a bullet heaven game, the screen is filled enemies that you are actively pursuing to earn xp and loot. In fact, a screen full of enemies is sometimes desired, because that means more efficient levelling.

Idk, take from that what you will. As any punk rock or metal fan will tell you, genre descriptions are an imperfect science at best lol.