Is it really? What he says is, basically, that Liszt not always played according to score, and that he changed stuff...?
And since that's all he says (and it's far from a secret), I would argue about the level of interest....
Interesting lecture. I have to say that I've heard the many variants for pupils ascribed in a related, but different, way to the one dismissed during the video - the version I heard, which sounds more plausible, is not that variant A was for pianist X because he couldn't play passage Z, but that variant A was for pianist X because variant A suited his technical strengths more than passage Z ie Liszt was purposely writing variants to showcase individual pupils' particular gifts.
Though it isn't mentioned in the Kenneth Hamilton lecture, something well worth looking at are the Liszt-Siloti editions which are based on hearing Liszt's performances of the compositions.