I think it's perfectly fine, if the article gave a definition it is using for mass shooting and used it consistently. The article (probably due to some weird wikipedia policy) uses incidents which were identified explicitly as a mass shooting by at least two of a preselected collection of 7 sources. Which is bizarre. It's literally more straightforward to select subsets of events which match a pre-made definition, than use events which are identified as X by external sources.

Edit: I just hate this sort of wiki-brain. The fact they consider off-loading decision making to external sources to be "unbiased" when literally less information is obscured to give a definition, apply it consistently, and use sources just to check whether the definition is met.