Can someone explain why is meta entitled to see a 3rd party's business plans, financials and statistics?
It's explained in the post. The FTC accused them of being anti-competitive. The only way to show you're not anti-competitive is to demand documents of your opponents that demonstrate you still have competition.
Breaking them up or taking actual action against them would require deeper investigation with FTC taking the lead rather than potentially handing sensitive documents over to the offending company.
If the company wants to fight the block of acquisitions would foot the bill for everyone being supeonaed along with the FTC's expenses regardless of the case's success.