Underwood coached a very white Stephen F Austin team for three seasons from 2013-2016. He only coached Oklahoma State for one year, which isn't enough time to overhaul a roster one way or another. This is his fourth year at Illinois: I believe he inherited the commitments/scholarship of his top black senior (Donsunmu) as well as his one white rotation player (Bezhanishvili).
I agree that he's made no overtures toward recruiting white players (and I mentioned this in my writeup), and I think it's kinda cheesy that his son seems to be the first guy off the bench in blowouts. Realistically though, the younger Underwood is a grad student walk-on who will leverage the experience to kickstart his own coaching career. This isn't like Drake Stoops at Oklahoma magically being the only white receiver the football team has featured in memory. Off the top of my head, Danny Ferry at UConn, Tom Izzo at Michigan State, and Juwan Howard at Michigan also coach their own walk-on sons. Boeheim and Iowa's McCaffery brothers are uplifting exceptions to the rule, being that they play meaningful roles on winning teams.
There aren't very many power conference teams starting three white players like Boeheim, and even fewer who made the tournament.