Board Game Arena

Created yesterday a 5p table for exploding kittens that was released...yesterday
One of the players has not played so far, I check the profile and "last seen" is 15 days ago, the day the account was actually created.
How does this work anyways and how to react?

I would guess they queued when they were last online and only now got a table matching their settings. They should have got an email notifications that their time was running out, so personally, I would give them a couple of days to respond, but if they don't turn up, you're free to kick them now that they're in negative turn time.

Game was released yesterday though,was it possible to queue beforehand?

Yes, it has been in beta (available to everyone) for a long time.