Games are allowed to copy mechanics. All fps games were called doom clones for a while, and they basically were. Go play hexen, heretic, wolf3d, doom, and tell me they arent all the same game. But because the genre evolved, now FPS games are really varied and good.
Slay the spire has some of the best mechanics of any recent roguelike so im eager to see people try their own take on it. The ONLY other deckbuilding roguelike I know of is Dream Quest and that game kind of sucks.
I can already see several differences:
The character creation system lets you pick 2 "magic" classes for your character. You can mix and match different decks together. That's pretty cool.
There are 6 magic classes as opposed to STS's 3.
There is a "regeneration" effect on a card which does not exist in STS.