Read the article, but it's FAR more nuanced than this. They are trying to make the argument that, essentially, because Musk has spoken with all these people and they like his plan, that he "owns" or has "controlling interest" in more than 15% of the company. It's a stretch because the easy counterargument to this is that his position is a very logical one, and he is just an activist investor trying to push forward an agenda that betters the company.
In the end, it all comes down to if the court agrees that an "agreement" by Delaware state law in this case reaches the level of proxy voting power (i.e. was Musk given essentially voting power by these groups/individuals). This is going to be a HARD SELL to a judge/jury. Unlike most civil lawsuit, corporate law is usually very narrowly interpreted, because these laws are so well-defined.
Legal experts are already weighing in, and don't think the Fund's chances are good:
Elon Musk’s $44 billion buyout of Twitter Inc. was challenged in a lawsuit by a Florida pension fund that argues the deal can’t close before 2025 because Musk was an “interested shareholder” in the social-networking platform.
www.bloomberg.com