When the pandemic started, China wasn't that paranoid about new infections, what changed now?
They locked down Wuhan early on. Foreign travel was severely restricted. I don't think anything really changed, they always had the intent to contain it completely.
New mutation or not, it's not sensible to have such strict measures, when the rest of the Western world is easing measures.
I can't argue that it's sensible - most of the actions of the Chinese government don't make sense to me, and starving their citizens is definitely one of those things.
But the immunity levels are most likely very different between China and the West where almost everyone had some strain of COVID.