They are cheating 100%. There are cheats for pc like legacy defending,5 * skills,5* WF all players, every player 99 stats and there was a cheat in fifa 18 that you could generate your scoreline. Offline no one will report so maybe they don't get caught.
It's literally impossible to win every game on legendary with 5-0, 10 shots on target, 60% possession plus and do it for 40 games straight without fail.
Doubt anyone is that good especially on pc. Most of the would be from Esstern Europe, probably farming coins most likely.