He is not wholly wrong, while not solvable in the general case by normal radicals, there is a family of functions, a special "radical" as he said, the Bring radical that solves the quintic generally. Of course as said its not a 5th root, but the solution to a certain family of quintics.
Nope. Wanna hazard a guess what theory was instrumental in proving that there's no closed form for general quintics?
https://en.wikipedia.org/wiki/Galois_theory#A_non-solvable_q...