Andrew, your claim is bogus because exercises do not work in isolation.
and of course, SOME bodyparts (ie the pecs, the abs) can be worked preferentially due to different inervation points and./or origin/insertion points
there is no such thing as a true "isolation" exercise. take the ubiquitous barbell curl. it doesn't just work the biceps brachii. it also works the brachialis, the brachioradialis, the front delt (depending), etc.
different back exercises work different muscles through different ROMS (and it is true that working a muscle through a shortened ROM preferentially strengthens the response (probably neural) closer to that ROM), and in different proportion
for example, a wide grip pullup emphasizes the lats to a greater extent vs. the rhomboids and biceps, than does a narrow grip pullup, or a chinup
and different exercises due to issues of lever length/mechanical advantage/failure of ancillary muscles have different sticking points, failure points, etc. that's why people can effectively use various exercises.
but, fwiw, the back is a very "wide area" (no pun intended) with a very large # of muscle groups. to work all of them sufficiently, one often can benefit from a large group of exercises. but i'm not sure it's 100% necessary, as i have seen some of the best backs ever on Olympic Style Weightlifters who only did 1) clean and Jerk and/or power clean or rack jerk 2) snatch and.or power snatch 3) squats
this is SUBOPTIMAL for hypertrophy, though, for a # of reasons (low reps, ROM issues, etc.)
but i disagree with your contention, for the reasons noted
you can't emphasize the lower lats, nor work them in a "greater" way, or however you want to say it
and again, there are LOTS of EMG studies on muscles. if it IS possible, it wouldn't be hard to construct an EMG study that would show this.
pecs? sure. and there is EMG proof
abs? ditto
lats? nope, and the reason why (imo) is because you CAN'T do an exercise that puts greater stress on the lower lats