They didn’t find any instances of cheating after August 2020. At that point his OTB ELO was only around 2450. It’s only after his last known instance of cheating that he improved into the truly great level.
They banned him after August 2020, and only reinstated him after a full confession the next year.
The fact that, after having being caught red handed in online for-prize tournaments and knowing he’s being monitored very closely online going forward, he suddenly starts rising at a world-historic pace (at an age this has never before happened at) at OTB tournaments suggests to me that having lost the ability to reliably cheat in one forum he turned his attention to another.
Regardless, there's obviously a huge difference between "no evidence he cheated after August 2020" (wow, guess his heart grew 3 sizes that month) and "no evidence he cheated after a 5 month ban which required him to beg to get his account back, made it very clear to him that they were watching him like a hawk, and they wouldn't let him play in tournaments even after that" (not exactly indicative of an unprompted improvement in his morals).
On the other hand, in OTB classical chess the difficulty is in bypassing the security controls. Players are much closer to engines, and far fewer moves appear to be completely inhuman. Furthermore, with an accomplice that is also quite good at chess, those lines could just be discarded. This makes it much easier to both play very good moves that are not all that suspicious, and take enough time to "sell" the thinking time.
This doesn't mean Hans cheated over the board, just that without very good physical security, any unscrupulous GM could gain 300-400 points virtually undetected, other than from the fact that they are playing much better than they used to. If security is a top priority, at the very least we need to do what the Sinquefield cup did in the last few rounds: No spectators and a 15 minute delay, so that all communication with computers/co-conspirators has to include the player relaying all the moves out before they can get any help.
This. And then let's test this Niemann character, as now I'm really curious as to whether -and how- he cheats at OTB.