[yesseri]

Niemann have admitted cheating before when playing online, so Carlsen is just not making this up about any random player. There is a history of cheating.

[diehunde]

Also Carlsen didn't accuse other players of cheating the other times he's lost

[lupire]

The sample size for that is absurdly small. (Carlsen only takes single classical losses seriously, since rapid/blitz is too random.)