Amazing. I could never imagine any person or corporation getting me to root for the ad industry.
I'm not not inclined to support anyone demanding more advertising as a point of lawfare.
Their claim is that a ideologically motivated organization called the Global Alliance for Responsible Media was used to create an illegal cartel agreement between advertisers to shut them out. I think there's merit to that, this wasn't just independent companies deciding not to advertise there, this was a coordinated effort, brazenly so, people were cheering about it at the time. There's a case to be made at the very least, let's see how it plays out.
I myself have been part of a company that provided advice to subscribers, it wasn't a cartel and not all subscribers followed the leads provided.
The Register is UK based but captured the essence of the whine at play here, Musk told advertisers to fsck off, they did, and now he is upset they did so.
That action itself will undermine Musk's case .. maybe it was a cartel action, probably it was not unexpected follow through in understandable reaction to Musk's attitudes and open armed welcoming and support of the kind of people that stoke far right rioting.
As you say, we'll see how it plays out in court.
Coordinating an ideologically motivated boycott is totally legal. Where they'd run into problems is if the coordination was meant to e.g. lower X's prices.
For example: Liz Cheney is someone I would never vote for (not that I have had to make this decision), yet I applaud her role in the J6 hearings, and her attitude toward Trump in general.