Source?

This comment is literally the first search result of "Chet Faliszek Valve employee handbook" for me. I've waded through several pages and haven't found a credible source of him saying that.

I don't have the precise link to hand (sorry) but Chet posts frequently on TikTok, I recall seeing him say this.

I'm sorry I can't provide you with one right now. Chet talks a lot about various things on his Tiktok, the story how he wrote this handbook as a recruitment marketing tool was one of the hundreds upon hundreds of things and Tiktok's search is utterly useless sadly.

That being said, I don't want to say it is a bad thing or entirely wrong. It is of course a brushed up glorified version that certainly took a thing or two from their real experience working at Valve, but it was not used as an onboarding tool until it was published (and to my knowledge, isn't used nowadays as they have since changed their internal team composition since Half-Life Alyx was released).