Most legal systems do this: if you're accused of doing X, and your claim that you weren't doing X hinges on evidence that can only be produced by other parties, you explain that to the court, and then the court can decide to compel those other parties to produce that evidence in the interest of a fair trial.
In this case, Meta is being accused of anti-competitive behaviour. Their claim is that there is plenty of competition and that if they are to be put on trial, then they should be able to present evidence that there is competition. The court agreed with that statement. Meta themselves cannot produce that evidence, because they are not privy to business goings-on at other companies, all they can see is other companies that are--in their opinion--competing in the same space. As such, Meta can only go "we consider the following companies our competition, their documents should make it ample clear that we are in competition" with enough of an additional explanation to justify each company listed. And then the court goes "very well, this is motivated enough to justify us compelling these companies to produce the evidence that you claim exists as part of discovery".
The only quirk here is the claim that a small company can't reasonably do what is being requested of them by the courts. And again: not by Meta, but by the courts. Meta doesn't get these documents, only Meta's legal team gets those documents, Meta employees don't get to see what's in the many boxes of discovery material that their legal team is going to receive. Not Bob from accounting, not Kelly from finance, and not Mark from the CEO team. Only the lawyers do.