To add to the oddities of it all, despite clearly being a DLC, it doesn't show up when filtering available GAs to "DLCs only": https://www.steamgifts.com/giveaways/search?dlc=true
The storepage itself is also lacking a link to the base game, as well as the usual banner stating that it is, in fact, a DLC.

You say it tells you "requires base game to play".
Can't say I see this message on my end, nor does a text-search on the page prove me blind for not seeing it... lol.

I guess there's always a chance this is one of those DLCs which DOESN'T require the base game.... there are a few of them out there.