Judging by the quality of their current squad, I agree that Chelsea isn't a contender for the Champions League title this season because some of their players are still quite young and inexperienced. However, if you look at the club's history, they have won the Champions League before, despite their performance declining significantly in the Premier League. Beating PSG in the Club World Cup doesn't guarantee Chelsea has a much better chance in the Champions League, as they sometimes play well and then miss opportunities to maintain consistency.
You are right that Chelsea young squad and their inconsistency are big issues for their Champions League hopes. But their past win showed they can surprise everyone even when struggling in league winning single Club World Cup match does not mean they will succeed in Champions League. That tournament demands top performance week after week something their young players still need to prove they can deliver.
While I agree that it's necessary to underestimate a team with such a long history and previous Champions League success, their Champions League opponents have much better and more established squads. Things look difficult for Chelsea, especially with a young and unsettled squad, but that doesn't mean Chelsea can't achieve great things.
This season, they have strengthened the right wing, left wing and offensive line, all of them are very young and good players.