Author here. Yes, as mentioned, all the colours and curves are approximations. I created it in a rush during my last summer break, to be able to submit it before the SoME4 [0] deadline.
replyI agree that working from RGB backwards results in a lot of back-and-forth between concepts, sometimes leaving unanswered questions. Starting with the SPD means getting to matrix transforms and chromaticity is a much longer route, but I feel is ultimately more intuitive — at least compared to most of the other ways to present it. Also, I wrote this for an audience with some (basic high school) background in math, but zero background in colours. So I'd rather it be drawn-out than underexplained. I myself didn't know much about cone fundamentals before deciding to write about it :)