Thats true; Manchester City cannot be guaranteed the FA Cup yet because there are big teams in the competition as you have mentioned. Based on the performance Manchester City displayed today, I dont think they can win the FA Cup if they keep playing like that because if it was a bigger opponents
Its obvious that if Manchester City played against a stronger opponent, then they might have not been able to win their match today, but even as Plymouth is weak team, they gave Manchester city difficult time in the match, Manchester City completely dominated the match, but they had problem scoring, until towards the ending of the match which everything changed for Manchester City, and they were able to score two goals plus the first goal scored in the first half of the match.
I dont think anyone is sure about the team thats going to win the FA cup title this season, Manchester City are not in form, and when they play against a team like Nottingham forest, Newcastle or Brighton. Manchester City might find it difficult to win clubs like that, so I dont really think we should be confident about a team yet.
Pep Guardiola and Manchester City management needs to strengthen their squad next season in order to be more competitive as they used to be.
If pep Guardiola doesnt improve Manchester city performance by next season, then they will be having a worst season, because most teams already understand the pattern in which Manchester City is playing, and they are no longer finding Manchester City dangerous.