Anyway, here's a request. I'd like to see your take on this one. I have mixed feelings about it.
Simply put, the theory hinges on the idea that Kass' teacher HAS to be seen in the game AND has to be identified. Which is entirely arbitrary and only based on writer meta.
Due to this, tons of little innocent statements get mangled into making Robbie sound like a jealous, bitter, lying man filled with regrets, something that does not fit him at all.
The video prefers calling Kass a liar over the simple explanation we don't meet or see Kass' teacher in-game. Which is dumb.
The theory clearly shows a clip of two male Sheikah present during Link's sacrifice, and we know that if Robbie is one of them, the other likely died, as there are no other male elderly Sheikah in BotW that could recognize Link.
Going by the video's standards, the obvious candidate for Kass' teacher would be the unnamed male Sheikah that, along with Robbie, brought Link to the Shrine of Resurrection.