His profile claimed he was a Buzzfeed reporter IIRC. Although he was being satirical, he didn’t indicate such, and this causes problems because the whole point of the checkmark is to signal that this person is who they say they are (including any claims in their bio). Since the average random person doesn’t even know who Milo is, they may see his verified profile and attribute his inflammatory comments to Buzzfeed staff members.
Twitter has many examples of taking enforcement actions for silly reasons, but this one actually made sense given that logic.