well no expert here... but I can relay what I've read, and that is that the emg version of a difference amp actually is running the two coils of a humbucker into sep inputs... then dropping voltage for anything they have in common, and amplifying anything they don't have in common. This keeps the noise floor really low. it requires some really underwound singles.
the schematic you linked to... that's just a garden variety booster. anything at the input is being boosted -noise included. Just like a pedal running after the guitar. Nothing wrong w that... just isn't anything like what is going on in an emg pickup (not being critical here... just observation). you could, essentially take any booster circuit and put it in a guitar. I've done it with micro booster, linear power booster, mid boosters, sho boost... and enjoyed it quite a bit. IMO it's a very useful and cool thing to do.