Just as the brace is a brace and not a stock, the magazine is a magazine. Just because some people use the brace as a stock doesn't mean it's a stock. If someone grips the magazine it's still a magazine. People grip the mag and mag well on their AR pistols all the time.this is good reading on the issue of the fore grip discussed earlier and the impact of over all length measurements.
Personally I have rifles and I have pistols and have never felt a need to have something in the grey area.
You run into interesting questions like is long Mag on a Charger really a Mag or could it be used as fore grip? I have seen people use it as a fore grip.