When you come back sober, the solutions won't make any sense.

Personally, weed might help in the creative phase of problem solving, but if it requires working memory weed is a big negative(at least how it affects me)

I find it's great for diving into refactoring projects that I've been putting off. I like to do that stoned because it's usually not that hard, just tedious moving files around and organizing things. Same way my wife reorganizes the furniture when she gets stoned.

I also sometimes find solutions to hard problems in my head when stoned, though I usually just write down the solution rather than coding it, as large-scale implementations are better done sober.

That would probably be a reasonable inference. He was one of the sharpest developers I've ever had the pleasure of working with.