Guess it depends on how you define 'mass shooting' but I don't think that statement is true.
My info came primarily from
Their source listed is USA Today article
https://www.usatoday.com/story/opinion/2012/12/25/gun-free-zone-john-lott/1791085/
A bit of looking, maybe I didn’t look enough the first time. I already had the page up, from a little poking around earlier.
According to the Crime Prevention Research Center, they deal in percentages. 1950 through June 2019 only 6% were not in gun free zones. Good article here -
It does seem to depend on your definition of mass shooting, they use the FBI definition of mass PUBLIC shooting...which differentiates from shootings in private homes, gang shootings, and shootings as a side result of other crimes. Also, most researchers prefer to consider it a minimum of 4 victims, not including the shooter.
A careful reading shows they consider the number of mass public shootings NOT in a gun free zone to be 8 since at least 1950. Still a very small number. They also go into detail on what is considered a gun free zone, not just a place with a sticker on the door, and the FBI definition of mass shooting.