Thanks for pointing out. If I click this DLC in the Steam software, it opens the main page of the store. If I click on another DLC, it opens the corresponding store site. Indeed very strange.

I can confirm that trying to use the key even though Steam page says you don't own when your library says you do own brings up an "already in your steam library" error on activation screen. I learned this the hard way by buying the DLC after mistakenly thinking I didn't own due to this strange steam store page inaccuracy...

Sorry, I replaced the key with another