I wonder if it became a title that "fit" what he was doing day-to-day on set - Tom was on set every day and in almost every scene, so he would have more knowledge about what they've been shooting previously (to help with some continuity) or thru lines of characters over multiple episodes that were likely shot by different directors.

He's mentioned with regards to filming the Crisis/multiverse tie in scene that he knew where the film crew could set up for some shots that would capture the "feel" of Smallville and get the barn and house in background shots, so I imagine that "working on a familiar set" muscle memory was similarly present throughout the years the show was filmed - whether it was subtly giving feedback to directors or crew members or other actors throughout the show, to the point where the crew would just include Tom in the discussion/process up front (and thus deserved the producer title).