That webpage link is a bit misleading. The mass flow rate will be limited to a maximum due to reaching the speed of sound in the tightest restriction, which is the point the webpage link was trying to make. Once past that restriction, the flow can very easily accelerate to supersonic.
This situation is called 'Choked Flow', see: http://en.wikipedia.org/wiki/Choked_flow
Also, take a look at these threads: