Submitted a resume via their website; someone replied pretty quickly (within 1-2 days). After a phone chat with HR, was given a take-home problem. After that, I went through a phone interview. Then a 4-hour in-person interview (2 technical interviews, and 2 interviews for fit). I came in once more to discuss final fit and salary/benefits.

Brief phone interview, hour remote technical interview, half day of (mostly technical) interviews at their office. It felt suspiciously easy. I would be wary of them using you for interview practice. That is the best explanation I can think of given that (1) they had a practice interviewer for an hour, and (2) their purported reason for not making an offer was that I did not have a specific technical skill, of which there was no mention until then. Other explanations are possible, but I would make sure to ask if there are any specific technical skills required. On the other hand, everyone I interviewed with seemed intelligent and nice. I think they would be enjoyable to work with. The experience was enjoyable, in a sense, but felt like a waste of time.