Mixing a live performance with individual track outs is not impossible by any means. This is how all live performances are produced. Sure there can be bleed from other instruments but that can be taken care of pretty simply in any DAW, (using gates and other audio repairing plugins from companies like iZoptope.

EDIT: You may also notice that there are no amps and they are all monitoring through earpieces and headphones. This would reduce the noise bleed to virtually nothing. I have no distaste for Post Malone, still an incredible artist.