Exhaust gas is indeed VERY compressible. Therefore his assumption that it can be considered as incompressible is very incorect.
Proof?
For example - look at some exhaust backpressure comparisons (you can find them on the net for some tests done on the SRT-4, for example) in turbocharged cars when they change the size of downpipe or turbine size.
More Proof?
Nonetheless, if it weren't true then turbochargers would not be real - as turbochargers convert heat energy into kinetic and in the process they drop exhaust gas temperature as the gases pass through them and the pressure on the inlet side or a trubo is typically 20-30psi higher than on the outlet side (taking one example in mind) - although this varies proportionally to the amount of boost being used.
