The found good enough Defensive players to rank 16,1,2,4 in points allowed and 14,1,6,9 in yards. Bills had a better D every year but last---which was close.
Bills also were a better Offense last year. Same result.
In the post season, the difference, therefore, is coaching.
The Bills, as you alluded to, have a 3-1 record regular season Josh vs KC. You can't cogently argue that the Bills have adequate players and coaching in the regular season, but that they need "better players" in the postseason, in order to get past KC.
That would make no sense.