From their about page:
We’re a passionate team that loves Team Fortress 2. We love it for its gameplay, story, its unique cast of characters and environments.
A few months ago, when s&box started to release access to the Source 2 tools and ways to create custom gamemodes, we started to ask ourselves: «how would TF2 look like if it was ported to Source 2?». We know that Source 2 provides a lot of graphical improvements as well as a new physics engine. It is a lot more up-to-date to modern game development standards, compared to the two decades old Source, yet similar in some ways. So seeing this game in a new modern engine really interests us.
And we know that Team Fortress community has been wondering the same, and now we finally have a good opportunity to answer this question: we have decided to gather a team of experienced game developers and begin our Team Fortress: Source 2 project, TF:S2 for short. This name is still a codename until we figure out the final one.
Together, we are aiming to recreate the Team Fortress 2 experience in Source 2 using s&box, with many improvements to the base game, as well as creating a truly new unique Team Fortress experience.
We want to take advantage of the Source 2 toolset and use it to its best to create this new experience.
TF2 isn’t open source, but I’m guessing it may be possible to use its textures through explicit consent from Valve if you’re using it for means other than to mod the game. Idk if Amper Software have already done that but regardless, this is like a separate gamemode for s&box rather than a game on its own standing (which is probably a different legal topic). Valve isn’t rlly known to sue project owners for technically infringing on their copyright, they actually have endorsed (under consent ofc) remakes before.