Hot take - I’ll wager China is publicly announcing this so Chinese citizens stop trusting Airdrop. Intelligence organizations usually like to keep uncovered vulnerabilities private so they don’t get patched. And if there is a vulnerability, Apple will 100% patch that shit.
By sowing doubt in AirDrop’s security, that might be enough to get Chinese citizens to stop using it. And if enough people stop sharing or receiving, then that is going to kneecap it as a channel for dissent.