They do. Even back in their pre-UEFI days, it was possible to flash BIOS from a properly-formatted USB drive by holding down a magic key combination at power on. But it was not exactly publicized as a supported method.
The sanctions aren't supposed to cripple China indefinitely. They're just supposed to give the US enough time to build fabs for military chips before China invades Taiwan.
And yes, I am aware that may not happen within our lifetimes. I did not write the sanctions.
They call it horror. But it's so much more than that.