Also, a mechanical primer would almost certainly be made of metal, and as such would not be consumed and have to be ejected somehow. The whole purpose of the G11 was that since it didn't have to eject brass, it could cycle much faster for three shot bursts. Electric priming is the only alternative that gets completely comsumed in the burning process.
The P90 does not have a rotary bolt. It does carry the magazine parallel to the barrel as did the G11, but it is a simple blowback bolt design. From World Guns:
The P90 is a blowback operated, selective fire weapon. It is fed from 50-rounds box magazines, made from transluscent polymer. The magazine is being located above the barrel, with the cartridges being aligned at 90 degrees to the barrel axis. Each magazine has built-in ramp that rotates cartridge to align it with the barrel prior to chambering it.
Notice how the cartridge is oriented on the spare magazine, in the middle of the circular end. (The P90 is a Bulpup design.)