U think I havent heard of this before ? Did you seriously think thats an irrefutable proof of imamate ? why did so many sunni books record it then ? why do so many non-imami sources accept that as one of the great merits of ali b abitalib.
And so many sunni scholars have had no hesitation in mentioning it in their books ?
btw over 100 companions have narrated this not just 76 and many of them were not even sympathetic to cause of ali ,
simply the hadith of ghadir was not a political appointment of a successor as it presented today by 12ers. then there are nawasib infested scholars like ibn taymiyyah who go in the oppositte extreme in denying it.Its correct context is best given by tahir ul Qadri in this book titled Ghadir.