Please provide proof that an airsoft magazine is a prohibited device. It does not follow that a component of an airsoft gun, which is not a prohibited device or even a firearm according to the actual codified laws of Canada, would itself be a prohibited device.
You're making the assumption that the magazine is a prohibited device.
Legal magazine capacity in Canada is determined by the type of firearm for which the magazine is intended. Since airsoft guns are not legally firearms, I'm not seeing how there can be a legal limit for their capacity.
Moreover, the legal definition of a firearm is still based upon the CCC, not case law and precedent. Until and if the Criminal Code itself is amended to comply with the findings of this particular case, airsoft guns and their accessories are still perfectly legal.