Well, yes but not easily: this API will indeed allow developers to more easily develop third-party clients for kbin, but I don't think it is a 1:1 reproduction of Lemmy's API, so it will require significant work for clients to support both Lemmy and kbin.
Also, do keep in mind that kbin and Lemmy do not have feature parity (like Boosting or following users which are kbin-only)
That's understandable, I figured it wouldn't be interchangeable with the Lemmy API. He did mention in his Discord a little while ago that kbin support is planned, so hopefully that's still on the roadmap now that the API is almost ready.