I think it's disingenuous to classify guns as defensive weapons even if they can be "used in defense".
Defensive weapons are armour, masks, shields or even boots with steel-capped toes. Shields and boots can actually be used as offensive weapons (like anonymity according to the GP) but are primarily defensive. They are primarily intended to deflect or mitigate harm.
Guns, knives, batons and even less-than-lethal weapons like pepperspray, water cannons, tear gas and tasers are offensive weapons. They are primarily intended to inflict (permanent or temporary) harm.
The defensive capabilities of offensive weapons consist of displaying them for intimidation (i.e. a threat to inflict harm, which is obviously an offensive act) and their use to inflict harm against an aggressor in self-defense (which is again about inflicting harm and therefore offensive). Their purely defensive capabilities are trivial (e.g. using a rifle stock to parry an attack in hand-to-hand combat) and at best secondary to their offensive ones.
Also note that every offensive weapon can be used to intimidate or in self-defense. A knife may not be very useful for intimidation in a gun fight and a megaton nuclear device may not be very useful for personal self-defense (unless you consider pre-emptive strikes an act of self-defense and even then you want some distance between you and the "aggressor"), so they could all be classified as defensive if we were to use the muddier definition you seem to suggest (i.e. defensive weapon = a weapon that can be used defensively).
Whether defensive weapons are (or should be) considered weapons at all is a matter of jurisdiction (I only know about the situation in Germany) but generally the idea is that defensive weapons are classified as weapons because they're only useful in a fight and you wouldn't use them if you weren't intending to get involved in a fight (this is why they may be illegal to carry/wear in a demonstration, for example).
This distinction may be a cultural thing, though. By this definition a "personal defense weapon" or even a self-defense weapon like a taser or pepper spray is actually an offensive weapon rather than a defensive one. This certainly works with the anonymity analogy (concealing your identity is not in itself a means to inflict harm, although it can aid in the success of doing so or in avoiding punishment), so I think this is what the GP had in mind.