The Chinese goverment now know that Sinovac, whilst not as good as the western vaccines, did an acceptable job against the early COVID variants. However its efficacy against the Omicron version is very marginal and they know it's back to a Wuhan 2019 situation if it gets in. They either develop another vaccine and administer it within a few weeks or lose face by adopting a more effective western version instead.
No wonder the borders are staying shut.
This might very well be the reasoning and matches the strategy they have followed so far.
Problem is, Omicron seems so much more infectious, I doubt you can keep it out, this thing just spreads too quickly. May be this will finally make them change course. They could be just trying to hold it out a bit more until they have adapted Sinovac.