This is definitely opinions based, everyone has one and what not, so take mine with a grain of salt.
You can't look at it by minutes unfortunately.
The first season I highlighted, Delph fell out of form/favor, and Mendy kept on getting injured. Zinchenko seized his position at LB and it coincided with City going on a run of 18 consecutive win run to reel in Liverpool and win the league. The second season I highlighted, he and Cancelo were being rotated, however towards the business end of the season Zinchenko was the first LB, as Cancelo dropped in form somewhat. Zinchenko in particular put in an amazing performance against PSG if I recall correctly. So minutes played don't tell the full story, I think any City fan you asked would acknowledge Zinchenko playing a crucial tactical role in those seasons.
Ratings are subjective, so let's assume your point of Chilwell and Shaw outperforming him is correct. The idea of him being a passenger in City's side is insane. You can't have a passenger in City's insanely high back line, without getting ruthlessly targeted by the opposition. Despite this I can't recall many instances of him getting mugged by top opposition, which suggests he has great positioning. Great in the air defensively. He has very good technique, has the tactical awareness to tuck in to support Rodri, or slot into the half space when Foden overlaps. How on earth is such a player being carried?