It is true. No team that is impossible to be defeated. Although Real Madrid has a stronger squad, it doesn't mean they will win the UCL trophy automatically. There should be a chance that they will be eliminated by other teams. Man City, Bayern Munich, and Inter Milan are the examples of few teams that look like very possible to beat Real Madrid.
I agree with you that Real Madrid is very strong. Even then it is not possible to say with certainty that the champion of the next season will be the Real Madrid team. Real Madrid have a strong and stable squad. When compared to all other teams, Real Madrid team can be considered as the most favorite for the Champions League title.
With Mbappe joining the squad, Madrid's forwards have been strengthened. If the bonding between Vini and Mbappe is good, Madrid's front line will be able to perform even better next season. But we don't always see the favorite teams win in the Champions League. Only in the 2021-22 season have we seen a mediocre team like Chelsea win the Champions League title. So even if Real Madrid team is considered as the favorite for the Champions League next season, there is no way to say for sure that Madrid will win the Champions League title.