I think you're reading a bit too much into that. If you're at a social event, when do you hear "Thank you so much for coming" from the host? Usually when you're leaving, or as the writer says, when it's time for you to leave.

But usually the host won't say that until the guests have made it clear that they are on their way out.

- "Honey, we should probably go" - "Well, thank you both for coming"

I have never attended a social event where the host would say "thank you for coming, see you soon" out of the blue.