I don’t have the time to watch it all, but I remember that the Steamworks Development channel on YouTube had recently-ish released this video about how games get surfaced to players and it also talks about what parts of the store are personalized and which aren’t.

In the video I can only see the small ad on the left side about the Steam Deck. You’re talking about the big banner ad that appears somewhere inbetween the sections? I can only guess they put it there for everyone, or maybe just every region that can purchase a Deck, for simplicity.