Yeah that number is outright false for "mass shootings". This figure originates from the approximate 33k people that are killed by firearms per year. Of that number, somewhere around 23k are individual suicides with firearms. Of the remaining approximate 10k deaths, 80% are gang related incidents. Source is the FBI UCR.
I think they're probably quoting the chance over a lifetime, assuming rates stay the same. According to Wikipedia, 382 people died in mass shootings in 2018.
Gang violence is often included in mass shooting statistics, and rightfully so, but depending on the depiction or commercial are not shown as such. On the other side of the coin, often times, gang crime is taken out of mass shooting statistical data, making it look far less likely, but more realistic to rich, white, suburban targeted audiences that politicians tend to advertise to. Neither is a lie, but both data sets are "massaged" to suit political narratives popular with particular bases.