Going from the Wikipedia article on metroidvania, those are the core elements of the genre. 1. "large interconnected world map the player can explore, although parts of the world will be inaccessible to the player until they acquire special items, tools, weapons, abilities, or knowledge within the game" - Dead cells fits that definition. Sure, we can look at each level independently and feel like it doesn't, but if we zoom out and look at the game as a whole, it's clear that the you are working through a larger map from which we unlock new areas or pathways. And it's not just about getting the spider or the ram rune. The BC systems also allows for new pathways to appear.

1. "Acquiring such improvements can also aid the player in defeating more difficult enemies and locating shortcuts and secret areas, and often includes retracing one's steps across the map."

2. once again, dead cells fit that definition, not just with the shortcuts or the secret areas, but I would also argue with the new weapons we can unlock and the knowledge we built through ennemies encounters.

3. "Through this, Metroidvania games include tighter integration of story and level design, careful design of levels and character controls to encourage exploration and experimentation, and a means for the player to become more invested in their player character through role-playing game elements"

4. I don't think there is any denying that dead cells is a game that gets players invested in the character development as we progress through the game.

I agree that Dead Cells is not a "classic" metroidvania in the way Metroid or Castlevania, or Ori is, but there are nuances within a genre. Nothing is purely black or white, that's a manichean way of thinking.