This seems to be the explanation of someone I care about very much. I want to talk to him about it, but I doubt that would be welcome or helpful. He seems content as he is, and that's perfect, there is nothing wrong with having this type of personality. He doesn't seem to be searching for answers, doesn't seem to be bothered by his nature. So why should I disturb him with a label?

For now, I will merely look into this and not project anything onto him.

What is already very helpful about this discovery is that I now have a clue how to interact (and how not to). I now understand his emotional distance and do not take it personally. I can stop digging to know him and accept that he has little to say, prefers intellectual friendship, isn't going to express a wide range of emotions, is probably always going to seem a little aloof, and it's ok. It's simply his personality.

Thank you all for shedding light on people who have this type of personality and how to communicate and share life with someone who has SPD.