I'm guessing they assumed that BSD would have a lesser chance of having drivers for the hardware in their router (or if their router is ARM-based: specifically

I'm guessing they assumed that BSD would have a lesser chance of having drivers for the hardware in their router (or if their router is ARM-based: specifically supporting the router's exact+particular system board and configuration of hardware) compared to Linux. A priori, I would make the same assumption; when I hear "router", I think "oh, that'll need OpenWrt".