I honestly don't know about Koki himself, but the term "concept art" is so broad that even fan art can qualify. And the OP didn't clarify on whether it's supposed to be produced in-house at a developer of any sort, so there.
Not really. Concept arts are for
concept.
Concept art is for development, is a study of characters, scenarios and objects. It's a useful tool for game designers and programmers to see what they should put in the game. Concept art is art design creation. It's large and complete scenarios which helps making reasonable level design. It's model sheets of characters to study his/her/its movements, expressions, colors, combinations, etc. They are also not only "art", as they have to combine with the design (architecture) of the game and characters personalities/background.
They are sketches and also finished works, but with development purpose.
The rest is illustration.
It is very different to go and buy a concept art book other then buy an illustration book.
As you might tell I prefer concept art books. It's a sad thing that the majority of concept arts and sketches never reach the public.