Buster Mathis fought better competition in his day than Buster Douglass ever did, and that would count for Buster getting that late round stoppage.
Assuming this concerns Mathis Senior, watch the footage of his bout against Chuvalo. That's the version which Jerry Quarry took apart, one who came in at a reasonable 234 for JQ [about ten pounds lighter than he was for Frazier]. Douglas was better against Tyson than Mathis Sr. ever was, and I think the earlier Buster would finally collapse late, like he did against Frazier.