The article about Counter Strike Source on the PCGamingWiki clearly states that:
Unfortunately, when testing it on a PC without Steam installed, the game refuses to work. The error it displays is "CD key authentication invalid for internet servers".
(To be exact, the game itself does load however an offline bot server is not able to be created, making the game useless).
Despite the various commands and tricks I used to try to get it running, the game would still not work. This includes things like setting sv_lan to 1 , running the game executable as administrator and also