Yeah, I can see why it could be confusing, but it'll also require to have the necessary relationship points with that one because if you don't have the relationship points, even when doing the swimming pool event, when Sophia is talking with him before noticing Amber "flirting" with Dylan, there's really no mention about any new lessons.
So even if doing the swimming pool event one can still very easily skip the lessons with Amber's son if not having enough points thus probably making it, as you said, to be a random event or something because you need to meet certain conditions to have that event instead of having it no matter what like the first time, making that event probably for those that are completely focused on that path. I don't know for sure, so I'm just guessing as you do.
It also makes me wonder if by sending Dylan to the boarding school and, in the second half of the update, if there's going to be a moment or scene between Amber & Dylan in the afternoon as he leaves for the boarding school instead of having the self-defense lessons.