I don’t understand what your point about TLS 1.3 is. It’s only relevant if you’re doing a downgrade attack (or equivalently, using an active middleware box). TLS 1.3 itself is not vulnerable to this because it (a) doesn’t have non-PFS suites to downgrade to and (b) protects the cipher suites by including them in the key exchange material. But if the server supports TLS 1.2, an active MITM can still downgrade to it if the client doesn’t demand TLS 1.3 specifically (which browsers do not by default). It won’t matter to China until there are lots of TLS 1.3-only websites (which hasn’t happened yet).

China was already leaning on passive DPI and L3 blocking before TLS 1.3 complicated (but as I said, did not preclude) downgrading to PFS ciphers. The reason being that for about the last 10 years, many sites (including default CDN settings) used SSL profiles that only allowed PFS ciphers. For such a server, downgrade attacks are already not useful to the Great Firewall, so adding TLS 1.3 to the mix didn’t change anything.

> So did they? Remember, I'm not talking about some hypothetical future, this technology is actively in use today and has been for some time.

Google Chrome (for example) will now use ECH if the website has the relevant DNS record - but it doesn’t use the anti-censorship mechanism in the spec to make requests to servers that haven’t enabled it look like they may be using ECH. This, combined with the fact that China can just not serve the relevant DNS record by default, means it doesn’t really impact the great firewall.

This is actually a good example of the non-technical side of this: Chrome could send a fake ECH on every request, like the spec suggests. This would perhaps make China block all Chrome traffic to prevent widespread ECH. But then Chrome would lose out on the market share, so Google doesn’t do it. Technical solutions are relevant here, but even the most genius anti-censorship mechanism needs to content with political/corporate realities.