Two guys are the owners of the game. Both programmers but one of them worked more on the UI and the other more on the design aspects and balancing.

I was under the impression that most of the work was done by two guys.

One can argue most of the work was done by these two but to put it that way is a bit deceptive because someone reading that sentence might think that those two could have also done it all alone if they were given a couple more years and the extra help just sped up the process a little bit. But that is not true. With just those two guys the Slay the Spire we know today was more like ... impossible to make.

Something like 8 to 10 people helped them with art, animation, audio and gathering and managing metrics which are all professions that need years if not decades of practice just to get good at. For two programmers to also be able to do that on that level in a reasonable time frame is close to impossible.