Magnus is certainly not a manchild, and has NEVER done something similar. He has lost in the past against lower rated players and has never brought up any drama about it. So ask yourself these questions.
An internet search of ten seconds confirms my memory.
Carlsen was in such bad humour when it was all over that he left the winners’ podium early during the medal ceremony. He looked thoroughly disgusted through the showering confetti and cheers. While some may view Carlsen as being sore loser, Karjakin was a gracious winner. “I understand Carlsen (and his reaction),” he told NRK. “I would also have been very angry.”
https://www.newsinenglish.no/2016/12/30/carlsen-loses-his-blitz-chess-title/
There are many others.
Being a poor loser is typical Magnus.
To this degree? Never. All it took was a tweet from Carlsen because he lost and is getting closer to under 2850 than the 2900 he probably dreams about.
So your premise is that it is surprising that an undercurrent of long held observations and suspicions breaks loose now? When should it happen? When he wins the tournament? When he wins the Grand Prix? When he wins the WCC?
Carlsen's sometimes-petulance is unsurprising, and largely irrelevant. If the tournament had progressed with Niemann taking the lead and winning, the result would have been the same by the end of it.