Maybe he will get away with it if he only cheated against Magnus, and only in that one match. Because it's perfectly reasonable for him to play against Magnus many more times and lost.

But if he cheated a lot more (as Magnus seems to allege) then he'll also lose to others he used to beat easily.

EDIT: I think a big issue with chess is this "perfection" mentality. Magnus cannot make a single mistake or lose a single game without it being a big show. Hans beating Magnus this one time, even though he was playing against black, should basically mean nothing - Magnus can probably beat Hans 10 times over. If Hans was beating Magnus 10 times it would be different - but also if he was on a streak to Magnus 10 times he would be playing with a lot more scrutiny.