Yes, you can only get the 4k broadcast by streaming it via the fox sports app.
I know what you mean about not being able to just flip through the channels. It’s definitely a minor inconvenience. I found a work around though. I have an external streaming device that plugs into the Tv (fire stick 4k) so What I do to flip between channels is just hit the input button on the remote and it takes me back to the last channel 8 was watching on cable, and I hit it again to go back to the game stream. This would only work with an external streaming device though, not the internal apps on the TV.
The only Bills game being broadcast in 4k this season is the upcoming Thursday night game on Nov. 25th.