Do you find this odd that there is no supernatural elements in the story other that the Titans? Because I do. I'm also sure we're given enough info to at least suspect Titans being a purely scientific concept. And the 'deal with the devil' is just a legend.
So, if that's the case, could it just be that whatever biological virus/organism/infection or whatever the ability to turn into a Titan is, it can only affect a specific race/subrace? And in that case, Ymir just spread it.
Again, my own speculation, but it does have some basis in-story.
All Subjects of Ymir are connected to via paths, and only they are connected by paths. Remember that the idea of paths started by realizing that Titan powers transfer to only babies among the Subjects of Ymir. The bones and flesh of a Titan body are sent via paths, so you need paths in order to become a Titan.