It's quite dangerous to make unsubstantiated comments and assumptions on US copyright law without the proper research.
Valve still owns the copyright to the game and just because they won't do anything now does not mean it is legal to redistribute it without their consent, especially when we know that the game is still being sold. [0]
They (Valve) reserve the right to enforce that and this site clearly does not have such a "license" and haven't disclosed as such. Why would you expect Valve to be in discussions with a 15 year old to redistribute the game for free?
So just say you do not know.
> just because they won't do anything now does not mean it is legal to redistribute it without their consent
I don't think the parent comment is claiming it's legal, other than the (unlikely) chance that this is licensed, just that it's up to Valve to enforce and not really our concern. A lot of cool things (like the similar https://noclip.website/) are prima facie copyright infringement.
> we can't know if it's illegal or not
I think we can.
We can guess this is unlicensed, and likely be right, but whether it gets taken down is up to Valve.
And I think we don't care.
> Why would you expect Valve to be in discussions with a 15 year old to redistribute the game for free?
Because projects like this are free publicity and don't actually compete with the product sold on Steam.