I’ve seen this sometimes in kids on the autistic spectrum. Splinter skills like being able to remember anyone’s birthday and tell you what day of the week it will fall on in any year, even in 100 years, etc. I think I once read a theory that this is due (in autistic kids) to an ‘eggs in the basket’ approach to neural power - they put all their attention and energy into a few things that hold their attention for whatever reason, and so can accomplish astonishing things in those particular areas. This appeared to be anecdotally backed up in cases where the children later developed language but lost these other skills.
It would be really interesting if the same thing is going on with this man but he learned to ‘turn it on and off’ so that he could focus on those areas and then return his brain to typical patterns for language and daily living.