Would China just absorb it? I mean I guess there is some benefit to having a buffer, but for all intents and purposes it would be China that speaks Korean.
Eh, I doubt that. Post-WWII, with the important exception of the USSR's Eastern conquests, major powers largely eschewed formally changing boundaries, or annexing parts of other countries, something which is illegal under international law. OTL, the PRC has held itself to the internationally-recognized boundaries, annexing only when there's an actual boundary dispute. They likely wouldn't annex territory that was universally (and even by themselves) acknowledged to have been sovereign Korean territory.
Instead, they'd puppetize it so it'd remain the DPRK or "Free Korea." Again, similar to Turkey and Northern Cyprus.