Well, L&P went through a lot of trouble to keep the two events separate. If the progress from the first event does not lead to anything more, then an exclusive branch could have forced a decision beforehand. I think, It's unlikely that the progress from the bathroom event will go to waste just because Dylan was at the party. He doesn't stand Sohia up, so there is no anger.It's certainly different with Ellie if he doesn't go to the party.The problem with that is,you have to choose between Sophia and Ellie event,you can't have both.
And if you go with Sophia event,probably that is the end for Dylan/Ellie route,and vice versa
So, Ultimately, he is able to make progress with both Sophia and Ellie that day. Exactly what is needed for a threesome. I see no logically compelling reason against this.