I would guess that supported for now means driver support within macOS but nothing more. I cannot see Apple's offering a EFI to flash onto RX5x0 cards for boot support now or in the future.
They're going to have to change something though. Right now, if you're on a pre-0089 firmware, the Mojave installer will insist on upgrading your firmware before you can install Mojave. But that firmware installation will silently fail if you've replaced your stock GPU with one of the two recommended cards (because they lack an EFI driver). So after the reboot, your firmware is not flashed, and yet the Mojave installer will keep insisting that you install the new firmware before it will proceed.