I guess the cool part about Metroidvania as a genre label is seeing part of the name of Castlevania being used in such a widespread manner. Why wouldn't Castlevania fans be happy to see this?
At the same time, this also reminds me a bit of the 90s, when all first person shooters were often labeled "Doom clones". Metroidvania is nowhere near that bad, but I also don't like how when you label a genre after a game, you always have to bring up the name of said game when discussing a very broad range of possible creative expressions, and that just doesn't sit well with me. Nobody calls first person shooters "Doom clones" anymore for obvious reasons, and I think exploration platformers as a genre has also grown into this huge, diverse thing, the same way first person shooters did. Yes, we owe it all to Super Metroid and SotN, but we don't have to bring this up every single time we discuss an exploration platformer.