Motion prediction (at least for regular moving character) is already included in UE4 and works fairly well.
The "not designed for MMOs" is referring to various other issues, for example, by default in UE4, if 1 players moves to another map, all players move. (This is typical shooter behavior, where the host of the match loads a new map, and everyone spawns in it.)
Another example would be the lack of any database connection code (most match based (and player hosted) games do not use a database for any of the game code anyway, so why add it?)
Now we already have a solution for these specific cases, but just to illustrate the kinds of problems you can run into when using a game engine designed for first person shooters/action games and try to transform it into an MMO. That being said, none of these problems are impossible to fix, it's just that you can't use any "out of the box" solutions.