By "cheating" he means candidates on the left and center were willing to drop out of the ranked choice race in order to prevent Nazis from being elected.
Just like America!
My memory's fuzzy; do we have ranked choice elections in the US, or ... ?
No rules or laws were broken in France, by the way. Nothing says you can't drop out of the race, nothing says you can't make agreements with an opponent to make sure the stronger of you beats the fascists and the weaker walks away.
It's "cheating" when they don't get their way. That's all.
Some states do have ranked choice voting -- at present, WA state does not, though we do have a ton of other voting protections and benefits, which is awesome.