Hmm...good point. However, a drummer still needs knowledge of rhythms, syncopation, and other rhythmic musical theory, which you don't need for turntables. Ask a drummer what the difference is between 6/8 and 3/4 is and they'll be able to tell you - a disc jockey probably couldn't. [Incidentally, the difference is that 6/8 is counted ONE two three ONE two three, whereas 3/4 is counted ONE & TWO & THREE]
I count 6/8 as One Two. If need be, I'll count it as one-two-three-one-two-three, but it makes more sense to make that bar 3/4 if you're counting each eighth note. [/sidenote]
Plus you're starting to get into profiling. You cannot say that a drummer or disk jockey would know that difference; it all depends on how he/she was taught. While a person may know this theory, does that mean that they can apply it? With a disk jockey, you don't really need to apply it, as the music speaks for itself, and you just manipulate it from there. A drummer should obviously know where the downbeat is and create his music from there. Again like you said, a person may not know what to call it, but the principles are there. This has happened to me many times where I cannot teach or explain what I just did. Nevertheless, I know what it is.
So instead of saying 'this person doesn't know what this is', you should say that "That person does not need to know this in order to be a disk jockey/musician/conductor/salesperson/broker/etc."