I do have a strong memory of it, having been an advertising media executive with friends at WJR at the time. I also lived through Bo's entire tenure as coach of Michigan. I was even a Michigan fan myself as a child because every kid growing up in Detroit in the 70s was a Michigan fan, and a Bo Schembechler fan. I'm just sharing my recollection of contemporaneous conversation about the events at that time, and the conclusion JR people came to is that Ernie won because Ernie was universally beloved, and Bo—to your point about divided loyalties, but probably more because of his general demeanor—was not.
Also, FWIW—I understand it's a huge apostasy to even broach this here—the JR people I knew at that time were not fans of Ernie in general. They thought he reneged on his promise to retire after his last contract, forced the station to give him one more year, and resented the soft power he wielded in that one press conference to curry public sympathy and make the station and the team look like the bad guys. They were personally glad to see him go. Believe it or not.