I think they are marketing this game as an action adventure for a lot of reasons:
A) It's more popular.
B) It communicate better to the public what the product is about.
RPG is a very vast genre in gaming nowadays. So it wouldn't really speak to the consumer if CDPR said it's just a RPG
I mean, Fallout, Baldur's Gate, Diablo, Crusader Kings, Vampire - The Masquerade Bloodlines, are all considered RPG by themselves, the game industry and game consumer, even providing a very different gaming experience, when compared to each other.
So, for the consumers that doesn't really want to dig in, and spend time finding out what the game is about, it's better to say that it's an action open world game.
It's easier to identify - that's why people think that this game will be like GTA V, and not like The Division 2 (That have more RPG elements in it).
C) RPG games that became very popular (Fallout 3 and NV) usually doesn't focus on action, because (with limited budget) the developers invest more in story telling (or so they say), so the final product end up having some clunky shooting mechanics.
Which is not Witcher 3 case, that have awesome action with great story, like a lighning strike that came from the sky in a sunny day. But it's "safer" to sell an action game, than expect the consumer (especially the new) to believe that there is second lightning strike coming.
D) I feel that the RPG fans are already in with this game (I preordered the game in september 2019) since they'd announced it. I'm expecting this game since 2013, and I can't wait to give this game a good old hub, nice and hot, spicy Cyberpunk, hehe boy!