That's right, also with Guardiola himself he doesn't want to take risks in this match so they rest key players on their bench, this is one of the strategies that is quite good even though they lost by one goal but what the Manchester City players have done is quite good only they lack in defense, also besides that we know their typical game is with ball possession so when their opponent makes a counterattack it is not strange that they are always off guard and this also happens to their main players.
if Guardiola plays their key players maybe the result is a draw or even they can win, but Guardiola has his own strategy which he thinks is the best for their team. Also by playing this second string squad it can be an anticipation for them to be better in the future.
Guardiola is a very experienced coach and with deep player resource in Manchester City, he has better opportunities to rotate players than other coaches. Nevertheless, it is not enough for him to try winning all titles in one season, and the injuries of Rodri, De Bruyne are warnings for him. I believe that Pep Guardiola did a great decision for Manchester City in this long season. Winning two biggest titles, Premier League and Champions League are enough for Pep Guardiola and Manchester City as a successful season.
They can win these titles but we see in Premier League, Manchester City have more challenge from Arsenal and Liverpool in this season while in Champions League, Real Madrid and Barcelona are not easy to be beaten in this season.