I've looked it up and it seems you are at least partially right right, though the book you link to is hardly authoritative on such matters as it is from a human perspective pre-WotLK.
But consider the following: "Blizzard representatives have stated that they feel the connection between Yogg-Saron and the Lich King ended up being poorly expressed in-game,[18] but added that it "had a lot to do with the ability for mortals to become corrupted.""
https://wow.gamepedia.com/Yogg-Saron
Add to that the immunity of the Nerubians to both plague and domination, his creation of the curse of flesh in the first place, Yogg-Saron being the "other" source of green glow, his direct wielding of plagues in combat, and his presence all throughout Northrend and general manipulation of the scourge.
It's by no means proof i'll admit, but most sources on other, well, sources of the plague are remarkably dated and notably nonspecific. "From the frozen throne" is open for much interpretation given that it effectively crashlanded on top of a glacier infested with Yogg-Saron's influence and minions, which curiously left him alone in his early days.