Ok. So the SAMD51 programs the 4809 via UDPI, and it communicates with it via serial. Got it. I was thinking somehow that with the SAMD51 acting the way it does (holding some boot program in flash), it doesn't have enough space to run programs, so it programs the 4809 instead which isn't bogged down by pre-loaded programs on it.
Nevertheless, like with the Leonardo, could you just build a board with the SAMD51 only and program it with Arduino software? Or does the software not support programming the SAMD51 directly?