Not that weird when you realize they are giving out an alternative to their game filled with microtransactions

Its worth noting that all of those fan projects were based off of an old leak of the source code. Shutting them down makes sense and honestly it was weirder that it didn't happen sooner.

Either way releasing this SDK gives the modders a brand new much more updated set of tools to make the mods all over again but this time they could even release on steam instead of just being some sort of semi-illegal back alley deal

Buying Campo Santo changed Valve a lot, for the worse.

Valve has made attempts to take down projects that follow those rules. TF2Classic is the example that comes to my mind since it's the main one I've played, but I know there are several others. In TF2C's case, they tried contacting Valve to find out why they'd been hit with a C&D, never heard back, and eventually quietly resumed development.

Some kind of miscommunication between legal and the devs? Maybe internal drama? Maybe they didn't really pay attention and still aren't aware TF2C is back up? Who knows. Still happened tho.

Also this is just source code, not the assets (at least as far as I understand it).

Could probably just require the official game to be installed too and load the assets from that directory, or might even be able to use the shared folder, but not sure if tf2 uses that, i remember l4d and css and hl2 used to.

Check out tf2 classic~