In my limited understanding, with zero-knowledge proofs. That's also what the blog mentions. Say, a website issues a challenge, you sign the challendge with your private key supplied by either your government (or some other authority). The signature is verified against a big set of public keys using a ZKP without knowing which specific public key matches.

I imagine the problem is more economics and politics than technology. Say, personhood verification becomes available. Then it is also that much easier to mandate age verification, and that becomes a slippery slope.