Hmmmm?
I must first declare that I CANNOT rap, love listening to it though, and am not a DJ, but I do know, in my FATAL attempts at "scratching" that it takes a lot of learning and practice to be able to sound good!
So, my answer is YES. I believe a turntable can be considered an instrument, you might not "play it" but you play things on it, which in turn creates something you are "playing", so yah...YES it is.
I have much respect for those who have mastered the art of using turntables. And they are so VERSATILE, anything...anything can come out of those things depending on who's behind the wheels... You go guyz (and girls) keep on making those beats and grooves that I love to nod my head to, and occassionally sing hooks for.
Play your INSTRUMENTS!!! I mean, because if you say, that a turntable is not an instrument, then that would mean that a DJ, is NOT an artist, which i think is blasphemous to even think.