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My guess is the "myth" started by being true
If you make a significant change to the VE of the engine the fuel calibration will be wrong which will lower tractive effort - power
For eg
Take a stock Mini classic with stock peashooter exhaust. If you swap that for a 2.5 inch straight trough and bolt on a K&N it will run like a dog until you rejet the carbs. The existing ones are simply not able to supply enough fuel to meet the increased airflow so the engine runs lean and makes less power
The same applies to all cars but to a lesser extent on modern EFI systems. With wideband lambda sensors and long term correction of calibration tables newer cars will automatically compensate for small variations in VE so will usually end up making more power
It also depends on the fuel injection system - map based will suffer more than maf as it relies more on correct VE tables for fiuelling
I'm pretty sure that BMW would have run sufficient time in the dyno cell to ensure there are no issues with their PE range
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