I'm not really disagreeing with your point but I think in general Valve has kept their games that share an engine on the same trunk, so TF2 should be on a pretty recent version of source. The update accompanying this release includes the bicubic lightmap and radial fog implementations from the HL2: 20th Anniversary update.

For Source 2 it ends up being a constant source of leaks where strings for engine features of in-development games get shipped out in Dota 2 and CS2 updates. We learned tons of stuff about Deadlock that way.

https://www.teamfortress.com/post.php?id=238643