This is in contradiction with the argument put forward by the lawsuit: that brands were willing to boycott X only as long as they were sure their competitors were doing the same. If it were true that brands avoid X because "it's toxic to their public image", then they should be delighted to know that their competitors are still advertising there!
Assuming that companies were just distributing their marketing budget elsewhere twitter essentially became an outside option for companies that were willing to bear the "risks".
Sorry, you're being deliberately obtuse here. If what the lawsuit claims is true (irrespective of whether it's legal or not), ads on Twitter are still worth something also to those who DO NOT pay for them, because otherwise they would not need to collude with their competitors to make sure they're also boycotting.
I'm not saying that this can be proved to be an anti-competitive behaviour. Just that it destroys the narrative of "ads on Twitter are a negative value for the brands that advertise there".