Except your argument is saying that guns aren't the issue since people had ready access to them before so its not what is causing the mass shootings. By all means ignore the rest of my post which was putting that in context too (which is to say that other places have that as a factor and yet, because they also do limit gun availability, they don't have the glut of mass shootings that America does, which nullifies your point).
I don't know if you were just trying to be a somewhat more rational version of the bullshit we've been hearing for years ("its not guns, its the media!!!!"), but a lot of people making the same argument you did, absolutely do try and claim that access to guns isn't a major factor (they'll often claim its not even a factor at all, although weirdly they often will, they'll just then claim that nothing can be done about it, there's already too many guns to be able to prevent easy access, or various other arguments that don't hold up to scrutiny).
Absolutely it is not the only factor (and all of the others need addressing as well), but it is the single factor that if you change it, would have the most immediate and be most effective at curbing this specific issue. Instead we've gotten the opposite. We've gotten more affordable, and more effective (deadly) weaponry, that is easier to access.
People focus on the guns, because its basic logic to go "hmm, we're having people shoot other people with guns...maybe we should make guns less accessible?" and then looking around when other places did that and it (shockingly, apparently to some people?) has tended to actually work. Hell even gun nuts usually admit that, they just claim that its too difficult or its too late there's already too many guns we'll never be able to reduce them enough to matter.