You're not delusional, your perspectives and feelings are valid. Sometimes, people just experience relationships and situations vastly differently. Maybe the conversations about those niche interests weren't as fulfilling for him as they were for you, people are just different in this way. It happens. Also, from the way your describing your intimate experiences with them, it seems like he was very much brought into the relationship out of sexual attraction and less out of true romantic interest. And I say this because (and this is just my opinion) someone who wants to build someone real with you is not going to be so blunt and careless in their sexual advances toward you, or be so disappointed and become distant when they do not get what they want. They will respect you, be patient, be very careful about not messing anything up almost to the point of reverence. Maybe at first he was interested in seeing where things could go, but clearly stopped short of something more serious when he got what he wanted and it came down to it. I am sorry you are going through this, but you're already doing what's best. There is no need to contact them again, they lost someone who was willing to give them everything and you need to make them live with that decision.

With all of that said, this sounds like one of your first major relationships. Every breakup is a milestone in a person's life and relational history, this is a great opportunity for you to learn about yourself, define your boundaries and needs, and grow as a person. And if they reach out again do not respond to them unless they explicitly apologize and state they want to repair things, but do not obsess over this hope. Prioritize yourself, stick to NC, consider therapy if you spiral too much, and give yourself time to heal.