I think if it went to court, aor an arbitrator, they would side with messi. Irrespective of the wording, they would look at the intent in the spirit of the clause and what it is there for, in an unforseen global circumstance.
i.e the clause is there to let messi leave at the end of the season, and to give barca time (the transfee window) to deal with it. given the season has just ended there, and the window is open til October, no judge would see Barcelona as disproportionately affected by the agreed clause being activated beyond the original date. Whereas enforcing the original date would completely screw over messi, making the move the clause is there to enable, impossible.
In my opinion. Based on nothing but logic.