The point is that Grothendieck, easily one of the greatest mathematicians of all time, who regularly proved deep and fundamental facts about prime numbers, cared so little about particular numbers that he accidentally gave an easy to see non-prime as an example of a prime.
He was used to working on completely different levels of abstraction, so when faced with concrete numbers he could easily make a mistake that a school-child (or hacker news commenter) could spot.
