Lots of people have the base game already, so fair enough tbh.

The Steam API (which SG uses) doesn't report DLC ownership. So any DLC or package that includes DLC will still show for people who own the DLC or everything in the package.

If you don't want to accidently join these giveaways just click the eye icon (next to the magnifying glass) to hide all giveaways for that game.

It's both like Channel28 says, and not.
It is true that Steam API does not 'report' DLC ownership correctly, so if this was a GA for any of the freestanding DLC's Channel28 would be fully correct.

But this time it is actually about how Steam handles game ownership, which Steam does not do at all. It handles only license ownership.
If you buy the base game and subsequently all of the DLC's, you will own one license for each product. One licence for the base game, one license for DLC1, and so on. But this is a pack, and it has it's own licence that does not correlate to the single purchase licences you most likely have. And therefore you can join even though you have it all, because according to Steam - you do not own it, as you don't own this particular pack-licence.

Enjoy! It's an amazing game (and DLC).