I can't say if Chelsea has improved in their performance or not, it's when the new season starts is when we would know. However, their performance in CWC is impressive and their fate will be tested in the finals when they play against PSG who have destroyed and humiliated top big clubs in various leagues in recent times.
The Premier League is a different ball game from the Club World Cup tournament, and it is more difficult than the Club World Cup tournament, so we can't use the fact that Chelsea managed to reach the Club World Cup final to justify or believe that Chelsea has improved in their performance, so I agree with you that it's when the new Premier League season starts that we'll know if Chelsea has truly improved in their performance.
Last season has been a proof that Chelsea is better than the previous season so I think it is a proof that Chelsea is better and regarding what will happen next season, anything is still possible and indeed sometimes there are things that are beyond prediction whether it is due to inconsistency or other problems but if it happens as seen in the Club World Cup then it clearly proves that Chelsea is better.