Sterling saying it's false doesn't mean anything. If he's going to say something publicly then he's always going to say it isn't true.
I'd be pretty surprised if this doesn't happen plenty of times a season and if Laporte, Cancelo, Aguero, Sterling, Foden, Gundogan and the rest haven't all had a falling out with the manager over game time. Didn't Foden get left at home during a Champions League away game last season? They're all competitive players and want to play. Guardiola knows what that feels like and how to deal with it.
Not saying it is true, by the way, and I'm sure it's been embellished by the time we're seeing people posting about it on twitter but I certainly wouldn't be surprised if the basic premise was true.