Closer role in the sense that he got saves and finished games but clearly not solely responsible for doing that. He pitched in over 100 games and threw 200 innings but only recorded 21 saves.
This SABR article has some interesting information on Marshall's season as well. It notes that under modern rules Marshall would have been credited with 30 saves that season, and that he only converted 64 percent of his save opportunities, which would be pretty poor for an out and out closer.
https://sabr.org/bioproj/person/mike-marshall/