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.

Their twitter page shares snippets of their WIP work, which look absolutely incredible even at its current state.