Magnus = undisputed #1 player for years now. He's Michael Jordan of chess. This 19 year old American player who is ranked 40th and has a history of cheating online faced Magnus in person few weeks back and beat him.
Instantly speculation of cheating started going around. The tournament said they investigated and believe no cheating happened but they decided to change the broadcast rules by extending the broadcast delay to like 15 to 20 minutes from live play to live television to make it harder for would be cheaters.
A week later they had a rematch online and that's the first tweet you posted where Magnus quits after the first move in protest of what he strongly believes happened in the prior week.
This

theory is the first time I'm hearing this though