Told by HR that technical portions were supposed to be a collaborative working session. However, reality was just someone giving you a question and sitting there in silence for 90 minutes.

Otherwise, seems like a great company with a terrific mission.

Question made more complicated than it needed to be. Once you strip out fluff the answer is clear. But, you'll spend 60 mins stuck on the fluff. Maybe that's the true purpose of the question. People that can get to the details that matter.