I'm not insinuating anything as I don't know anything...
But late december dutch TV aired a documentary where Dumoulin raised doubts about his performance in the Tour, reportedly saying "I'm having a hard time understanding. I don't see how I could have gone 90" faster".
(sorry, source is in french, couldn't find a report in english)
Dans un documentaire diffusé par la chaîne de télévision néerlandaise NOS, Primoz Roglic, dépossédé du Maillot jaune à la Planche des Belles Filles, et Tom Dumoulin, 2e du chrono, s'interrogent sur la montée impressionnante de Tadej Pogacar.
www.lequipe.fr
I was disturbed by this comment because it was very similar to the comments Greg LeMond had made after the Luxembourg ITT in the 1992 Tour de France regarding Indurain.
I don't know if Dumoulin doubted his own abilities or other rider's and how much of a factor it was in his decision but... it seems likely this disbelief had an impact on him.