No way was he referring to the "2019 draft class if they all stay four years." Not a one of those guys on the list will be in school four years. The most logical way to interpret "Tanner is #5 on their draft board for QBs" would be their active QB list if they entered the draft today.
Regardless, if the list was for seniors in 2019, then Mangum being 5th is not outlandish; if it is for current QBs eligible for the draft, then it is an inconceivable leap.