Basically, what they did is record in real time at the concert using multi track recorders or mixer with outputs for plugging in all individual microphones from the stage including instrument mics. It is also evident that there was a final mix down as you say, they added rhythm guitar separately, mabanga and off course the fake crowd. It's exactly what Ferre did on his Paris acoustic show, Koffi also does that with JP Kys that's why most concert dvd's have high quality sound sometimes even better than what the crowd was actually hearing.
