Comment by stephen_g
RGMII requires way more work to run board-to-board (heaps of signals, quite precise length matching, impedance control, etc. on the boards, better board-to-board connectors etc.) and at the end of all that will likely be less robust than just running Ethernet. I'd much rather use SGMII just because it's far fewer signals to match (even if it runs way faster) instead of RGMII.
The chips they're using might already have Ethernet PHYs built in anyway which might also be part of the reason they're using Ethernet.
Eh, it's just a half dozen signals in each direction running at 125Mhz DDR. That's in spitting distance of being able to be bit banged by something like an rpi pico.