This is an enigma. If you have never used a powder extinguisher, you will be unaware of the consequences of doing so in a very confined area, i.e. in the cockpit.
The powder that is used is extremely fine. The rate of discharge from any powder extinguisher I have used (quite a few) is at least an initial huge delivery followed by forceful flow lasting until the thing is empty.
The trouble is, even when they are fitted with a trigger, supposedly to control the flow of powder, the actual amount of control available is about useless in a situation we are describing.
The upshot is this. To use a powder extinguisher in a confined space such as an aircraft cockpit is in my own opinion hazardous in the extreme, simply because the effect produced is to suffocate the fire, which it does very effectively. But it will also suffocate the occupants of the cockpit, or at very least render them incapable, very quickly.