In fairness, Thornton is the active NHLer that's behind Marleau in regular season GP. The abbreviated seasons aren't too helpful though in that regard. I thought it would be something for Marleau to attain that record in Leaf colours. I feel the same way about Thornton now. Doesn't seem likely, and I certainly wouldn't prioritize it ahead of a Cup win, but...
The last time Thornton played in 82 games was during the 2015-16 Season, which coincidently was the year San Jose made the 2016 Stanley Cup Final against Pittsburgh.
However since then he played 79 games in the 2016-17 Season, 47 games during the 2017-18 Season, and 73 games during the 2018-19 Season.
Although during the 2019-20 Season Thornton did play in all 70 Sharks games and baring a late season injury he would have played in all 82 games.
The last time Marleau missed a regular season game was during the 2008-09 Season. So those games he did not play for the Sharks last season when he was still an unsigned UFA did not count as any games missed, so his consecutive games played streak is still active.
Just like you I wouldn't prioritize the all time regular season games played record over the Leafs winning a Stanley Cup and I'm sure it's something Thornton does not care about.