If the guest has availed of the pay less up front facility for a stay of fewer than 28 days then Airbnb takes the second and final payment from them a few days before checkin. The guest is advised of the payment schedule at the time of booking so it looks like asking them is the only way to find out for sure.
As you've seen, the terms and the help system articles don't give a definitive answer. They just say that it will be be sometime before checkin.