But Mann made a mistake. The book he was likely quoting from, 'Science and Secrets of Early Medicine' by Jurgen Thorwald (which, to be fair, is not referenced at all by Mann) does mention the Ebers papyrus in the paragraph after the quote (on pp. 57-8 for people playing along at home) but the willow quote itself in the paragraph before turns out to be from the Edwin Smith Papyrus, Case 41 to be exact. It can be read here:
https://archive.org/details/oip3_20220624/page/374/
So that quoted willow did exist in ancient Egypt.