No not really: Mahrez came to City as a league winner and star in his own right; Grealish was a celebrated player and the star at his old club; De Bruyne was on his way to superstardom well before he landed at City's door; Sterling already had a fantastic campaign at Liverpool and so on and so forth.Sorry but what you just did in this whole post is judge the finished (or almost finished) products City players have become, not what they were before playing under Pep.
Their players are interchangeable because of the system. The players definitely aren't equally as good as each other and it often shows despite having a very distinct and functional system in place, but the system maximizes every players potential.
The main star of the City team is Pep and his system.
They are interchangeable because they were already good players who have, no doubt, been given a boost by Pep, but a bunch of player who have been collected from clubs across the country/continent who nobody could have amassed with such depth.
This specifically pertains to the league, where, fatigue and injuries are huge factors for most teams and they will see a sizeable decline in output or performance without one or two key players. City is the only club this does not apply to - they can even lose their best player and still be challenging for the title - Pep's coaching is a fraction of the reason why they don't have single point failures, the remainder is obviously in the fact they have an absurd depth to their squad at league level.
The reflection on Pep is what he does with that same team in the CL where the advantage he goes into any league campaign with is gone. And there, he's just another manager in the pack, or certainly not the seemingly monstrous conqueror he is in the league. You cannot sweep this under the rug; Pep is unique for a number of reasons, but the football manager-esque squad is a huge component in how they barrel through the league without needing any one player to have a miracle season.