It's a regularly used debate tactic of the right. He was going to imply that whatever number the student came up with for mass shootings was inflated because the criteria was too vague and erroneously counts gang violence. He would then have used this to simultaneously downplay the seriousness of mass shootings and suggest that trans people represent a larger proportion of mass shooters than they actually do. It was his way of discrediting the argument that the student was making; that trans people are actually underrepresented in the population of mass shooters. The reason for the student making this argument - if you don't know - is that Charlie Kirk has blamed trans people for mass shootings in the past, jumping on the conservative bandwagon that always transvestigates mass shooters because they believe they are a trans conspiracy.