I don't agree. I think Far cry is an open world game more than an FPS.
FPS i categorise the somewhat linear first person shooter experience.
The thing is, this trying to say they are expanding the genre is a huge mess. It just makes everything difficult to categorise and you are calling FPS's RPG's at the end of the day. We have to be more specific on our scrutiny or we end up with one category for all of them (wich action-adventure is becoming), wich just defeats the purpose.
As a general rule, whenever you're unsure on how to classify a game, I think the best advice is to take the approach Nintendo took with classifying Metroid Prime and look at what the core focus of the gameplay is. For Metroid Prime, it was adventuring, hence it was given the title "First Person Adventure," despite being played from a first person point of view and having almost all of Samus' offensive capabilities be firing projectiles.
So I agree with Nem here; hack and slash was a phrase that originated in order to describe the original Devil May Cry when it released, so it's obviously referring to the type of gameplay found in Devil May Cry. Which begs the question: "What does Devil May Cry focus on?" Hitting people with swords, yes, but more than anything it's about fast paced combat with an emphasis on juggling, long combos, and encouraging the player to be stylish. Perhaps most of all, it was about encouraging the player to be on the offensive as much as possible, which was something of a phenomenon for 3rd person action games at the time. Onimusha has none of these; the combat reminds me of a slightly faster Dark Souls more than anything.