That's not true. He was originally charged with a felony that was dismissed. Multiple articles written on it. Here's one that mentions it:

https://www.nbcnews.com/news/us-news/michigan-state-university-gunman-turned-evil-mean-mothers-sudden-death-rcna70579

He plead guilty to a misdemeanor which does not prohibit you from owning guns, and his probation ended in 2021. He was legally allowed to own guns.