I believe that you have missed the potential significance of Xi's use of massive authoritarian control. Not using vaccines could have been a test to see whether his control would give him a critical advantage over other countries lacking his control. Were a weapon so powerful that the rest of the world could not control it with vaccines, China might come out the most powerful country in the world. If so, then he has learned that the strategy would not work.

Regarding your other arguments, Xi controls what the rest of the world knows about China. He could hide whatever he wanted to.

You seem to be implying that I am "bloody stupid." You are wrong.

Its not a stupid thing to do when:

1) You're facing a demographic crisis due to your aging population and your next closest rival has a population of which half are under 30,

2) the virus primarily kills the elderly,

3) your approach to increasing influence and power is to increase your relative position and your relative position is decreasing after decades of increasing, and

4) you can cause an economic crisis for your rivals when they shut public life, and consequently their economies, down, and thus increase your relative position in the world.

I won't comment on any western motivations for going along with it.

That's not definitive because you're assuming motivation, it could be that someone didn't want a vaccine, and you're assuming all research was publicized to us. This was the fastest vaccine ever developed in human history, it's worth noting.