And if you read the posts after that one, you'll also see that the article is severely outdated on the subject of 'average over turns' vs 'average over number of points'. So since BTS, it fully functions like you would intuitively think it would: If you gather 50 prophet GPP, 100 Merchant GPP and 50 scientist GPP during the creation of a 200 GPP's Great Person, then the odds are 50/200 = 25% to create a Great Prophet or Great Scientist and 100/200 = 50% to creat a Great Merchant no matter how you got those great person points. It doesn't matter what great persons you were generating during one turn vs the next or how many 'sources' of great persons were used to do it. The only thing that matters are the total numbers of great person points of each type created in the city during the construction of the great person.
Ooooh,, thank you for that update.