I'm not really a stats guy, but I've heard and read that Matthews is actually pretty damn good defensively by pretty much all metrics. Like miles ahead of the rest of the top centers in the league, apart from Barkov maybe.
It should be Barkov's, he's probably the new Bergeron in that he will be nominated pretty much every year and win multiple trophies during his career. It's a reputation trophy for the most part, it's hard to define defensive play purely based on stats and those who vote have no time to actually watch enough of potential candidates.