The ultimate incentive to contribute is that maintaining your own separate fork of development (and backporting/merging upstream changes) eventually becomes too challenging for most organizations.

Now, for an organization operating at China scale, it's possible that a fork could be sustained (so, the "China Syndrome" becomes the "China Fork"), but it's likely still not an optimal strategy, and for mundane areas there could still be substantial leak-back.

So long as changes aren't distributed "outside the organization" (quotes applied in context of defining "organization" relative to a Communist state in which nominally all property is State property), legal GPL code release requirements aren't triggered. Though you and what nuclear-tipped army plan on enforcing that against Beijing is of course another matter.