The discussion isn't about Barcelona being the best ever, but they were the team at semi finals, and seeing how Inter played on the final game, we could safely assume Barcelona could play better than that if they were at the finals. It would be realistic to expect them to not lose 5-0 on the final game.
However, it would also be realistic not to expect Inter to get that bad result normally too, that was their bad day. But saying a team that is at semi finals wasn't good would be unlikely and should be more inclined to say they were pretty good at that moment as well. Flick has done some great improvements on the team, and if they could get Nico too, then this team will be one of the best in UCL next season as well.
Football will always show us very Different scenarios that we do not expect, a team that is about to be relegated is capable of beating the one that is in 1st place in a league, anything can happen , we are people who now do not have 100% certainty to say that a team will win, about 10 years ago we did have it, but now we have to look at the fact that many teams have improved a lot , sports betting has become difficult for that reason.