Most weeks on the PGA Tour, when there is a 36-hole cut where at least 78 players make it (used to be just 78 professionals, but now amateurs count too), there's a secondary cut after 54 holes down to the number nearest to 70 as possible. All of those players who make that third-round cut get to play the final round. Everyone who misses the secondary cut and get MDF'd, they get paid and a FedEx Cup point for their three rounds of effort.
However, in the FedEx Cup playoffs, there is no threat of a secondary cut. Though both The Northern Trust (125 players max) and the Dell Technologies Championship (100 players max) have 36-hole cuts, and, in fact, had secondary cuts up until 2015, that has gone out the window.
PGA Tour policy makes it clear: The Northern Trust and Dell Technologies Championship do not have a MDF in the field, even if literally every player somehow made the top 70 and ties. That has evolved because players' seasons could come to an end as a result of a MDF, and the PGA Tour deemed that a bad idea. So, they did the right thing and got rid of the secondary cut in the first two FedEx Cup playoff events.