I remember reading a co-interview he had with Miyazaki, and while the former seemed to speak clearly and comprehensively, Giraud seemed to be speaking as if from dreamland, with the interviewer and Miyazaki frequently not quite sure what he meant(!) But maybe that makes perfect sense for one such as he.

I can think of a possible reason for that. As far as I know Giraud regularly utilized drugs. For example he developed his own fast drawing technique that included microdosing Cannabis during work. I also heard first hand, from someone who personally met Giraud once, that he was quite fond of LSD.