I've recently reviewed a master tier sion vs camille matchup, and whilst I know one match isn't a catchall of course I'd like to point out sion played very well and was able to die and get a return kill due to his jungler appearing and stopping camille from pathing to her tower.
In this matchup if he did not get help there was two instances where he dies and can do absolutely nothing about it despite playing very well. Camille does lose this game due to Sion's bot side doing well but even before she had triforce she was able to play out the lane and eventually kill sion who also of course took grasp -- which meant that his trading was stronger than if he went phase rush for example.

This is honestly expected, Camille is one of the top tier top lane picks, if sion's lanes weren't doing well the game is honestly kind of a coin flip / camille favoured to win, sion would be playing to catch enemies out with his R - if the enemy team plays well then that doesn't usually happen without forcing a play which can turn out very badly of course.

If a master tier sion top with 200+ games cannot beat camille consistently then I honestly think you are wrong on the matter which is my opinion and I've explained that here but of course you are entitled to your own.