As an American I can confidently say: no, knowledge of guns is not universal, even in parts of the country where gun ownership is common. And there are plenty of people who own guns but are completely incompetent in their use.
As for Japanese martial artists' knowledge of the sword, what is your metric? Willingness to be filmed waving a sword around? Or actual training to some degree of competence? As with guns, there can be a wide difference between real and perceived skill level.
I see you are veering what I say towards competence. Still yet to find a Japanese of said era in said martial arts who didn't have a level of competance with bokken.
But I feel that's not your point, your point appears to be solely a bugbear with my bokken knowledge and competence.
First differentiation needed is that it can be used frequently to demonstrate principles. Those who don't know this will see what they want to see.