Separate games mean nothing. Maybe PSG was exhausted by the previous games plus everyone predicted an easy win for PSG and this led to the burnout of the PSG players. But even if Chelsea is really stronger than the best team of the season at the moment, this does not automatically mean good prospects ahead. Bookmakers quote Chelsea's chances of winning the title as 1 to 13. I think you will agree that against the background of City, which went into crisis, the fresh champion and Arsenal, which have passed their peak, this looks like a very weak quote.
PSG getting exhausted by their previous games is not an excuse for the defeat they conceded against Chelsea because Chelsea were also exhausted by their previous games, but they were able to beat PSG in the Club World Cup final. Chelsea beating PSG doesn't justify that Chelsea will perform well next season, but you should allow them to enjoy their win against PSG while it lasts.