I think there's a very real chance Buffalo's week 18 game against the Patriots also gets flexed...to Saturday.
They have to move two games to Saturday (and one to SNF--Seahawks/Rams has to be the clubhouse leader for that, possibly Vikings/Lions, though it's possible that game will be best played at same time as Philly), and ideally you'd like to move games to Saturday that are important but by themselves can't clinch anything. I don't think that will be the case for any week 18 game right now, though perhaps the Philly game would count if the Lions/Vikings is still for the North.
So if the 1 seed is off the table, moving the Bills could make sense, but even if it's still possible, they may bank on the Bills probably winning to keep the Chiefs game important, plus the allure of showcasing Josh Allen again. (Or they could just put Chiefs/Broncos as two playoff teams, and possibly render the Bills game completely meaningless).