It is good to see someone still discussing about the issue since I was not more often hanging around at itch.io, only for some special games exclusively here. Itch.io still allows company or dev provide only Steam serial key here without a Drm-free version. As you can see, such as this one, provides Download Instruction.pdf. https://shuyansaga.itch.io/shuyan-saga
It is just like you said, Itch.io isn't really meant to be a Steam key purchasing store. I feel that it is true that many indie games (small team with huge creativity) are connected with Drm-free, it is a very similar ideal for every games should be individual and unconstrained with DRM (a stupid thing to restrict games and gamers) I am sure many people who support Itch.io must care for Drm-free too. But maybe itch.io is not so ideal to be praised as a great kind indie game platform, it is always about the money to freely allow them to do that.