I can imagine it's audio given that he stated he didn't want a mish-mash of audio tracks all with the same "amount" of each drum on it.
For what I know, something like this would be rather tough and would require first and most importantly, EQ'ing to remove the major frequencies of the instruments you don't want (or bring out the freq's in the instrument you're working on).Usually though, even with EQ'ing, you're not going to get rid of ALL of, say, the sharp punch of a kick drum or a snappy snare when attempting to isolate it from a hihat.
Following EQing though, some certain steps with side-chaining
could get *decent* results.
But I'll bet some other folks around here have much better insight on and experience in something like this.