>Roaming works to bypass GFW. Apparently many local employees of international businesses have a Hong Kong SIM card with generous data allowances, just so they can communicate with the outside world.

Isn't that basically just an IPSec VPN?

Usually, yes, but it's an IPsec VPN that the great firewall will explicitly not mess with in any way, because they don't want to inconvenience foreigners who are there on business.