He says in the link it’s specifically the C guide, the rest of the website is fine. Though... yeah, DDG queries like “beej c guide strlen” give reasonable results for me, if with an unjustifiably high-ranked position for the mirror at http://docs.hfbk.net/beej.us. Bing ones only include the mirror and the other guides (and a Scribd-hosted PDF copy, of all things, as the first result below a huge navigation card referring to https://beej.us/guides but without the C guide among the links).
Incidentally, what is the "legal status" of Scribd hosting a partial preview of works that they tell you are BY-NC (attribution/non-commercial use only) and telling you to become a member to be able to view the whole thing? Is that not a commercial use?