The following scenarios have the final game of the year being for the division:
Bills go 3-0, Dolphins go 0-3 (Bills 10-6, Dolphins 9-7)
Bills go 3-0, Dolphins go 1-2 (Bills 10-6, Dolphins 10-6)
Bills go 3-0, Dolphins go 2-1 (Bills 10-6, Dolphins 11-5)
Bills go 2-1, Dolphins go 0-3 (Bills 9-7, Dolphins 9-7)
Bills go 2-1, Dolphins go 1-2 (Bills 9-7, Dolphins 10-6)
Bills go 1-2, Dolphins go 0-3 (Bills 8-8, Dolphins 9-7)
So out of the 16 possible ways the next 3 games can go (ignoring ties), 6 would have the final game of the year being for the division. If you remove the possibility of the Dolphins going 3-0 and the Bills going 0-3 (which I think is pretty reasonable considering their schedules) 6 out of 9 possible scenarios involve the final game of the year being for the division. The only cases where it wouldn't would be:
Bills go 2-1, Dolphins go 2-1
Bills go 1-2, Dolphins go 1-2
Bills go 1-2, Dolphins go 2-1
It seems more likely than not to me that the final game of the year will be for the division (which I'm sure is what they envisioned when they crafted the schedule that way).