The idea is that someone else has to support in midfield. If you've pushed up both full-backs into high attacking areas you no longer need your wide attackers to provide width, so one or both of them can come into the middle. In this case that would be Pogba and/or Greenwood. Both those players prefer to be in central areas anyway I think. In our case, the FBs didn't push up so Pogba couldn't come fully central and Greenwood stayed wide, meaning we had big gaps in the middle with a lonely Fred there and an overcrowded defence. But that doesn't mean the system is objectively bad, it just means it wasn't done right. After all most teams will leave three players in nominally defensive positions unless they're really chasing a goal. Whether its two CBs and a FB or two CBs and a DM, its the same number of people committed to the role. Its not like we're losing an attacking talent by playing Matic as one of those.
The real question is whether Ole wanted him to do that or not, and if he did, why no one else joined in.