I'd read that as 'the whole truth' rather than a suggestion that the Surely You're Joking series contained outright lies. The books (obviously) don't go into all the details of Feynman's personal and sexual relations, so there is surely much that is omitted. They also don't exactly convey a sense of extreme veracity, being structured as collections of amusing anecdotes.
Collections of amusing anecdotes is exactly what they are. Feynman didn’t write the Surely You’re Joking series. They’re stories he told that Ralph Leighton wrote down and collected into books.
