Dev's quote quoted in the article:

After revealing POSTAL: Bullet Paradise, a title Running With Scissors was planning on publishing but not developing, we’ve been overwhelmed with negative responses from our concerned POSTAL Community. The strong feedback from them is that elements of the game are very likely AI-generated and thus has caused extreme damage to our brand and our company reputation.

We’ve always been, and will always be, transparent with our community. Our trust in the development team is broken; therefore, we’ve killed the project. We have a lot of good things coming (some you know and some you don’t).

Bullet Hell is used for games where you need to avoid complex patterns of screen-filling bullets.

Bullet Heaven was coined more recently to describe games like Vampire Survivors where the opposite is true: the screen is still full of bullets, but you’re the one shooting them.

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.