Yeah, I see something similar, have them all but can still join.
This is due how Steam deals with licenses. According Steam, you don't own games, you own licenses (for the games) and as a bundle like this many times has its own license (instead of being a bunch of licenses for the individual parts of he bundle) the Steam API will report back that you don't own one or several parts of the package even if you actually do.
On this one it seems like the DLC's actually come with their regular licenses, but the base game comes with a ''Anniversary Edition X'' license compared to the regular Arma 2 license which is why all DLC shows as owned for me, but not the base game.

This is true for almost all bundles/packs sold on Steam, and is why it's always a good thing to check before entering pack GA:s.