Bad luck for them, they should have taken the deal.

Trademark protection is to protect trade. A company that doesn't make a product yet is at a huge risk trying to hold a name against an established company that wants it... You can only infringe a trademark if you are doing business in the same or a very nearly adjacent industry (Apple Records v Apple Computer as a historical example).

I'm sure they can litigate it if they want, but when the question comes up of how Facebook being named Meta interferes with the business Meta of Chicago was engaged in, what will their answer be? If they weren't planning to do something in either the social network or virtual / augmented reality spaces, the likely answer is "neither of you are infringing the other, go about your business."