I don't think that's true. All third spaces I can think of are based on some activity, around which some community (or individual social relationships) form which might or might not be so closely related to the activity.

- Church/temple/mosque/etc: worship - Bar: drinking alcoholic drinks - Gym/sport: physical exercise - Volunteering: whatever you're volunteering for - Coffee shop: coffee? Reading, working?

All these have "a thing you do other than socializing and meeting people". You could (and do) go there specifically for the activity without socializing and meeting people (just like church).

Spaces that are "social-only" are pretty rare. Coffee shops are maybe closer to that as you're probably not going to consume many coffees, but people stay to read, work... it's a bit less structured than other third spaces (and personally I find that it makes it more difficult to socialise there)