Sundin wasn't quite on the level of guys like Modano and Yzerman defensively in his era. But he was dependable and could be much up against top centres. He played on the PK frequently and scored a lot shorthanded.
I think some people don't understand how much of a positive impact he made at Even Strength. During his prime in the Dead Puck Era, he was one of the best ES scorers in the League and he was on the ice for a lot more ES goals for than against despite often having mediocre-esque linemates and getting tough match-ups (Toronto didn't exactly have a Norris candidate on the back-end, either).
Second in Even Strength points between '97 and '04.
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That's fair. It seems like his excellent ES goal differential has more to do with his puck protection abilities than his defensive acumen though. Sort of like how Jagr's puck protection has given him a solid +/- rating throughout his career despite not playing a lick of defense. But I guess that's also a point in Sundin's favour that his playstyle affects the ES goal differential positively (I don't really know Marner's stats on this)
I still find it pretty noteworthy he didn't get much selke votes as a center playing in a large market, dependable sounds about right for Sundin's defensive acumen. The one time he even had a vote thrown his way during his prime, the team was 10th in GA/GP and 3rd in PK% in 2002. Considering Marner is one of the few wingers in the league to regularly receive selke votes, I don't think it's all that far fetched to say he has the edge in actual defensive play.
Also, the current iteration of the leafs aren't exactly defensive stalwarts either. 2022-23 the leafs were 7th in GA/GP and 12th in PK%.
In 2021-22, 19th in GA/GP and 8th in PK%
In 2020-21, 7th in GA/GP and 24th in PK%
These are the years with Marner's highest selke finishes.
In terms of giving up goals, current leafs weren't really better than Sundin's teams. And yet, Marner has still received tons more recognition for his defensive ability.