Barcelona having a great offensive line wouldn't be enough if they didn't had a great defensive line as well. Hansi Flick made some massive improvements on defense, he realized that the defensive line itself isn't good enough, and if they just parked the bus they would lose, so even in games where a tie is enough, they still play counter attacking at worst case, because they just prefer to make that a unison work that would allow them a chance.

Whatever it is that makes them so much better, it is not really what the yare after, what they want is to shadow the problems at defense until they get the title. After they do, and maybe open up some financials, they could get some defenders which could help the team immensely for the future.