No. Valve has nothing to do with this.
Valve allowed them to use their engine, to clone their game and use their store/distribution platform, that's not "nothing to do",
No. You can still download it for free.
But it's $19.99 on STEAM. And if EVER a "mod" deserved it, this is it.
yes you can download the old mod as released in 2012 (it would be impossible for them not to allow that anyway), but they are charging for updates and Xen, something which was planned to be free as the 2012 release, but OK, I really enjoyed the original mod, so I hope they do well... I just dislike the whole paid mods thing and the change of plan... I guess I'm from a different time, like when I bought HL1 15 years ago and I can still play CS, DoD and other amazing mods (with all the updates they received once they became standalone games, well the ones using the original engine that is) for free, being an Hl1 and Hl2 orange box owner I was happy to play this mod for free, but the guys that made it did and excellent job (I'm judging just by the original release, not this early access one) and they deserve the rewards, but also the whole $20 now and more later is also silly