From an article from chess.com. This is printed in the article, verses the quotes from the comments section down below.
Online chess fans and commenters were quick to speculate. GM Hikaru Nakamura theorized that Carlsen withdrew because he suspected Niemann of cheating in their game the previous day, saying: "I think that Magnus believes that Hans probably is cheating." - Wow - and from Hikaru ...
In an interview after the game, Niemann mentioned that he prepared based on Carlsen's use of the g3 Nimzo-Indian against GM Wesley So in London 2018. However, this game does not exist, and Chess.com has not been able to locate any other modern game by Carlsen in this line.
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Here are some comments regarding this article from chess.com.
Initially I was in doubt that Niemann was cheating. Afterwards I saw the interview he gave to Alejandro Ramirez just after the game with Alireza finished.
Alejandro is my new hero. He disactivated the engine and asked Niemann to show lines and variations of his game against Firoudja. Niemann was in panic and was throwing random moves like a patzer. Alexandro Ramirez, who's rating is much lower than Hans Niemann was showing much better understanding of the potential variations. Please watch this video before claiming that Hans Niemann is innocent.
Another comment:
We know for sure that Hans lied. He had not prepped that line based upon Magnus' prior games. So why the lie? It's extremely suspicious and very likely that Hans did cheat. Further investigation should be conducted to possibly determine how. That fact that Alejandro Ramirez illustrated Hans' inability to replay the lines shows further likelihood of cheating.
In response the St Louis Chess Club will be increasing scrutiny of potential irregularities:
After the round began, the Grand Chess Tour announced that they are taking additional anti-cheating precautions, including a 15-minute broadcast delay and increased radio-frequency identification (RFID) checks.
Wow!