Your explaining the difference but not explaining why it makes a difference.
To matters of gun regulation, of safety in public spaces, of trauma to the affected, of national reputation (pick any one, or all, or something else) why does the intent change anything?
I'll start off: To have the intention to mass-murder purely for the sake of mass murder could be worth isolating and studying because that is a specific and extreme psychological problem worth solving. However, not all mass killings (with intent, for your sake) will have that psychological trigger at root. A religious or racial extremist, for example, is different than a disaffected teenager.
In this circumstance, intent is interesting if one is interested in those other things (psychological issues in American youth, the spread of religious and racial extremism), but ultimately are secondary issues when it comes to measuring gun violence. A mass stabbing by a racial extremist, or a teenager blowing up their high school with fertilizer would still need to be measured.
You are complaining about this organization's yardstick, but I don't hear a compelling alternative from you for this specific measure. You are saying they should be measuring a totally different thing, which is arguably irrelevant to this measure.
Are you saying that we should have Allowlists vs. Denylists for types of gun violence that are acceptable? This seems to be the fundamental premise upon which we disagree....
From my POV, intention is immaterial because there are no 'good' gun deaths, so splitting hairs has no values.
It sounds to me like you're saying if you go to a mall and have a mass shooting in a totally sober state, that's bad, but if you get hopped up on bath salts and then have a good old fashioned shotgun rampage, that's ok and we shouldn't count those ones....