You are wrong. First of all, La Liga rules state that a player can reduce his salary with a max 50 % from the previous year; Messi agreed to do that.
Secondly, the club can max have 70 % of their income spent on salary. Barca stands on 95 %, as of today, without Messis salary, hence theyre having to offload players, still.
So if you do the math, they would basically have to get rid of Griezmann, umtiti, pjanic, coutinho and sergi roberto to sign Messi. Laporta tried just that and failed. Nobodys taking on their salaries, theyre too high, especially in full. For example, Pjanic is probably going back to Italy on loan but Barca will still pay probably half his salary. Every team in the world knows that they have to reduce salary by any means so there will be loans where Barca still has to pay a percentage of the salaries.
They couldve taken the cvc investment deal with La Liga but then they wouldve been forced to give up TV rights for 50 years and making a superleague impossible for them to join. So they chose not to and thats why Messi couldnt be signed.
La liga were not going to make an exception because they want to force Barca to take the cvc investment deal, so that they cant leave La Liga to form a superleague in the future.