I think you both are -- They're saying it holds up with either definition of 'mass shooting'; 1. Four or more people killed in a shooting. or 2. Four or more

I think you both are -- They're saying it holds up with either definition of 'mass shooting'; 1. Four or more people killed in a shooting. or 2. Four or more people shot in a single incident. It's just an either-or, but the 'or' is listed as an 'and' since they're enumerating the two definitions of mass shooting. E.g. It holds up with definition 1, and it holds up with definition 2. I agree a CS major would write it differently but I wasn't confused fwiw.