What prove do we have that Chelsea is very strong to the point that they can win UCL. Winning PSG in the CMC finals is not a guarantee that Chelsea have improved and stronger than the rest clubs that will participate in the Champions League competition. Don't forget that other clubs too have become stronger than before.
Winning the Champions league is not something that is impossible for Chelsea because they have improved a lot and was able to defeat and outplayed the best thing in Europe last season to win the Club World Cup so that is prove enough to say they are capable of winning it although it won't be easy at all because we have a lot of strong teams in the competition. You should understand that winning the Champions league doesn't mean the team is the best and strongest among all teams, we have seen some other clubs win it when they are by far the best team.
Chelsea is a much-improved team, as we saw in the last CWC tournament. They crushed Paris Saint-Germain in the final. It wasn't a simple, fluke win; they dominated the match. With these qualities, it's clear they'll be one of the most feared teams in this year's UCL. A team that easily beats PSG can beat any team in the world.
You're right they dominated against PSG and scored 3 goals. PSG was at their best form and Chelsea was able to dominate them so they are definitely a strong team now and can challenge for the champions league title next season