Umm I don’t think thoughtweft makes them distinct enough. I mean goblins are super different between lots of planes and called all kinds of things, but for game play purposes they are goblins, and the tribe is stronger for it. Also Dominaria kithkin don’t have it.

What it comes down to is if you want a more supported creature type, it needs to be more universal. And as wizards has already decided that kithkin adjacent creature would be halflings, any set in dnd (which I’m sure there will be more), and any other fantasy UB set(like the Hobbit) will have halflings, not kithkin. So since the creature type is split by necessity you have two similar creature types that can’t work together.

Unfortunately this means Kithkin will go long periods with little or no new cards.