Gun rights aren't for stopping active mass shooting events. Gun rights are to protect yourself and you small circle of family because the police are always too far away.
Active shootings are bad for regular people to try to stop because usually those people who do, end up being killed by the policemen they finally show up. A regular guy with a gun can never be expected to rush into a school to confront a shooter.
A regular armed citizen will be charged with a crime if they stop a school shooter or any other spree shooter in a gun free zone.
This data is disingenuous because they are plotting a unicorn event with a normal event to prove that Unicorns aren't helpful. The question doesn't make sense.
It's not disingenuous because it's answering what lots of right wing people say about mass shootings instead of gun control. "Why don't we arm the teachers, why is it a gun free zone " etc. This is the answer to that question, not your statement.
Your criticism assumes the person with the gun is responding to the attack, running toward the sound of the gunshots.
Concealed weapons aren't for responders. Concealed weapons are for the targeted, intended victims; the people already present when the attacker begins.
This chart includes only those scenarios where a criminal attacker was not stopped before firing their first shot, and was not stopped until they had continued shooting long enough to be grouped with the rest of the attackers on this chart. It includes only people who were allowed to continue their attack long enough to qualify, and does not include attacks that were prevented entirely, or were stopped before reaching the chart's threshold.
The chart also fails to address one of the main reasons why so many of these shooters decide to stop shooting and run away: how many of them saw guns in the hands of their intended victims, and left before those victims fired a shot?
It also doesn't make any distinction between events that took place where the intended victims were allowed to be armed or not. Of course there will be less instances of armed defenders in areas where arms are prohibited.
OPs premise is akin to the "small government" advocates who ruin government services and then point at how they don't work.