Yes, that's right; I generally pop back in to macOS every so often to check for OS updates (which update the firmware as well - no way to just update the firmware).
>>vulcan+(OP)
I seem to remember there being a tiny recovery partition or cut-down version of macos or something maybe from boot CD (we have an old iMac).
>>mixmas+Ii
There is a recovery partition (in fact, it's used when installing Asahi) but I don't think it's possible to update the firmware from there. Boot CD might work. You could try asking on r/AsahiLinux.