the wastegate is not a diverter
the turbine has same very high rpm as the compressor - what is the turbine's effect ? (wastegate-open)
the exhaust is not a smooth flow - there's a lowish mean velocity carrying sonic velocity 'pulses' in both directions
who knows what's going on when the wastegate is open ?
and tbh - who cares ?
isn't Prof.Limebeer etc an applied mathematician not a designer of compounded engines ?
given naïve 2014 design predictions and data he presumably made a good job of predicting the best modes of engine use