Especially with music such as this bleed can be your friend, but lets take a look at some of the factors that might helping this setup work. I'd be willing to bed they are getting a greater amount of isolation than it appears.
Looking at the room they are in, I'd be willing to bet there there are not a lot of reflections to deal with.
1) I'm guessing here, but those black spaces between the boards on the walls and in the ceiling are probably black fabric covering insulation or some other sound absorbent material. If you look closely you'll notice that the boards are spaced unevenly. The larger gaps allow low frequency's to pass to the absorbent material, the smaller gaps work for higher frequency's. Having a variety of gap sizes allow for dampening of a variety of frequencies. The cool thing about these things is they don't totally dampen the room. The wood part reflects, the black part absorbs. A totally dead room is not always a good idea, I think these sound better.
2) Placement. Being directly across the room from each other the backs of the microphones, orchestra and drum mics, are facing each other, and if they are using predominately cardioid or hyper-cardioid patterned mics, and it looks like they are, this will add a degree of isolation
3) Notice the bass player is in the iso room with the piano. Bass generally carries a lot of energy and is therefor more difficult to isolate.
4) Close Micing allows for a great amount of isolation simply because whatever source you are micing at about an inch will be louder than anything else around it. As for the times a particular source is not playing it is fairly easy to trim the dead space.
5) Lastly, ( I promise ) it does not get much better than the BRBB. The easiest mix I've ever done was a one off live gig with a similar big band from Kansas City. Put the faders up and let them do their thing. They were so good that I would have had to work at screwing things up. They don't make mistakes. Bad notes are not in their vocabulary. So editing is almost non-existent and who cares if it bleeds a little, the performance is freakn' awesome.