what would be the purpose to hide it?

To avoid further retaliations from USA that might prevent their progress.
And to not disclose industry secrets.

I am probably on the pessimistic side and you maybe on the optimistic

True, I’m an optimist on their behalf, because China has done such impressive progress already, but I also know this is way harder than sending a man to the moon.
I agree 100% about restricting the tech is a huge mistake, and yes physics are indeed the same everywhere.
Hopefully when China figures it out, consumers everywhere will benefit. The Political implications however, is an open question.