I think so too...
In Italy it seems that all the athletes are competing to admit touches and for me it's stupid and has nothing to do with sportsmanship: as you well said, there are two referees and a video check system and the athletes' task is to play...
I completely agree. It is referees job to see those and judge. I mean why should Knollema go and admit a mistake just because it was at a crucial point and the opponents were complaining? I am sure there were like 10 other net touches throughout the match, referees did not see and players did not admit, or many other errors. To me it feels silly that a player is expected to do this. The only exception to me is when there is no challenge option and referee asks the player directly, then I would hope the player to be honest but still it is not wrong if the player lies, since clearly everyone else lies a whole match, at least once.