Oh, no, you’re not getting it wrong! (At least to my knowledge.) As I understand it, demiromantic people don’t develop romantic attraction until they form a close emotional bond with someone, and honestly, I can absolutely see Dean being demiromantic. I like the headcanon, personally, and I can definitely see how it would work. After so many years of knowing Cas and getting friendly with him, Dean might start to develop feelings he doesn’t completely understand. I like it!
But, just to throw this out there, aromantic people can be in “romantic” relationships, and they can be very happy and love their partners, it’s just not romantic love. They can still be happy, it’s just a different kind of love and a different kind of expression.