Even the article talks about this. The backdoor is when there’s no E2EE - he speaks of the “absolute right to privacy” which the proposed age verification solution wouldn’t breach in any way, so it’s actually the loss of encryption. It’s when a legal and technical framework are put in place to do age verification which can be switched to identity verification in a blink of an eye. It’s when cryptographically attested software is the only way to use the internet which even the author, as a suporter of the upsides, knows what it implies on the downside.