The readme at the linked Github repo mentions that it probably is exploitable from a running OS if you implement your own EFI_BASE_PROTOCOL.Communicate() function.
I'm assuming that would probably need admin/root access to exploit, but it does mean that you can turn an OS-level privilege escalation bug into something substantially more persistent and difficult to recover from.
If you can get someone to run a USB that'll boot you open the user up to a bazillion exploits already and already own the machine.