Please allow me to restate:
The basic argument given earlier was that those who didn't like guns in their game were anachronistically wrongity-wrong-wrong, as firearms were supposedly common in the time when Europe had heavy armor like D&D uses.
But now you're saying, "Well, they don't have to be common". This undermines the original logic for their presence in the fantasy realm. If they don't have to be common, matching the real-world, they don't have to be present at all, either!
I think you're going to have to show your work on tht last sentence. In the real world, firearms ranged from common to uncommon, contemporary with advanced steel making (rapier/katana/articulated plate). They do not range from common to nonexistent. You can argue "different physical laws," but not if the physical laws permit a number of other explosive effects.
You can either ignore the discrepancy, which is not wrongity-wrong-wrong, but is senseless. Not necessarily bad, but not necessarily good, either. Or you can rationalize it, which has the advantages of not having guns, while not being seneseless, either. There are reasons for each approach. I think rationalizing the situation is definitely the ideal, but may not be worth the effort for some campaigns.
And here's the thing. You can introduce any element you want by fiat. If it is an anachronism, it will be fine with anyone who is unaware. It will also be fine with anyone who is aware, but does not find it jarring. But if someone is aware, and finds it jarring, that's a disadvantage in creating your imaginary world.
There's that King Arthur movie. I could forgive a lot, but not its claim of historicity. After all, it was set in the post-Roman era, the "Dark Ages," yet it included Lancelot. Lancelot, as we know, is French, and was introduced to the mythos in the 12th century. It's pretty much impossible to not notice Lancelot's presence, which becomes all the more jarring every time they throw in "historic" details, like Arthur's still-anachronistic but slightly less anachronistic allegiance to Pelagius. It makes the whole movie, irrespective of its merits, smell like bull pockey. If the movie had been called, say, King Zarion, and took place in Zinoland, it would have merely been a lightweight action film with lurid yet shallow commentary on civic order.