Purely anecdotal, the fat bikes that I encountered on the return were too far back to finish on time.
I got to Brest around 23:00 Monday and slept til 6:30 am, had a nice breakfast, so, I was in daylight all the way to Tinteneac where I also slept the night. Watching oncoming riders is cheap entertainment. I hit the massive bulge coming up the Roc and then until the next control. I did see one Fat bike at the tail of the bulge. Then, it started to thin out a little into Loudeac but still saw outgoing riders into Tinteneac! It seemed to my eye at the time that the certain bikes and groups were going to have a hard time based upon mental calculations at the time.
I seem to recall reading a blogpost somewhere about a fatbike finishing successfully.