This is probably due to it having files marked as "linux" in the database, but not being released to the public...or they have a beta branch with Linux support (like Indie Game: The Movie), though that largely stopped happening.
I've passed this on to the regular Steam team.
As this isn't a Linux issue, it's a web store issue, this won't be tracked here. Also, the featured titles have changed so Insurgency is no longer there. That doesn't guarantee it's fixed but the right people know.
Nothing extracted yet.
For some reason Insurgency has showed up on the Featured section on the store home, even though it's not on Linux.