Because in my opinion, during Barcelona's heyday, many opposing teams also implemented bus parking, and so on, but because Barcelona's effectiveness at that time was extraordinary, it was not a problem. Flick must realize this so that they do not lose points again when facing weak teams that use a defensive strategy with the term bus parking. Now, with a big points difference with Real Madrid, of course Barcelona, if they want to catch up with Real Madrid, must not make the same mistake again.
They have to balance well between attack and defense and in latest matches of Barcelona, it's so clear to see their defensive players did not complete their jobs well. With many conceded goals, it's very hard task for their attacking players to score more goals then conceded goals. It's not a style of a winning team or a champion team, because a team can not always score more goals than opponents, and if opponents score many goals, chance to equalize the match or beat opponents is very small.
Barcelona attacking system still works very well, and they only need to enhance defensive system to reduce conceded goals, then they will have better opportunity to win matches in future.
I think Barcelona have an equal balance in attack and defense at the moment. You might want to differentiate these two aspects of this team based on their recent match performances. If we look at the statistics of their recent matches, Barcelona is ahead. So if a team performs a bit poorly in a match it does not prove that they are weak in attack and defence. But in the matches they played against Real Madrid and Bayern Munich, their defensive players were very strong and they performed very efficiently. As a result, those teams were not able to score many goals against Barcelona in those matches.