The article quotes Feynman’s letter as saying “I am not sure how I did it, but I goofed.” What he meant is that he doesn’t know how he made such a simple error, not that he doesn’t know what the error was.

Like if I wrote in an article or book chapter that 2 + 3 = 7. I’d know what the error was when it was pointed out to me. But I wouldn’t be able to imagine how in the world I made such an elementary goof.