What if, due to the absence of one of the players, they didn't play within the first 15 days? But they could easily play the game within the remaining 6 days. Even 2-3 days, if they wanted to. But then, suddenly, the other player has some problems, and what happens then?

Oks has already answered; I'll explain the idea behind the regulations.
To be precise, the schedule is as follows. The regulations state that players must notify the organizers 5 days before the end of the round. The rounds end on Fridays. The judges wait for these notifications on Mondays. And if, as you say, one player has been absent for two full weeks plus two days, i.e., in addition to the working days, there are 3 pairs of weekends. It would be unfair to force the player who has been waiting for this opponent to play a match within 5 working days. Therefore, the absent player is declared the loser 5 days before the end of the round. The exception is when both opponents agree to resolve the situation themselves.