Just because something is “technically” possible doesn’t mean its scalability and costs are a actually considerable option.
Any mid-range / price firewall solution is capable of effectively blocking most VPN solutions. Both OVPN and Wireguard VPN traffic is trivial to identify as such and block. Here’s an example and another.
Btw, I’ve never seen something like that, my VPN worked even in China, and that must mean something…
China’s great firewall works a little bit differently. They aren’t actively blocking certain kinds of traffic by default because that would mean a large DPI effort they don’t want to undertake. Also if you google a bit about it you’ll find that people’s experiences are mostly “my VPN worked fine for a day/week/month and then it was blocked”. It seems they’ve some IPs and domains blocked and the rest is some kind of machine learning that applies rules as it sees fit, this guy here has a good analysis of it.