Jon Heyman is a national sportswriter -- not a local guy. Why would he call out the Marlins specifically if there are "many, many teams" interested? While it is possible that Heyman thinks the Marlins are the franchise that will attract the most interest in his tweet, to me it sounds more like the Marlins are seriously interested, and there's a number of other teams with tepid interest. But the agent wants to make it sound like the market is bigger, so they leak this tidbit to Heyman.
Heyman asks, "Who else interested?" But the agent can't say, for instance, "The White Sox have knocked the tires," if the White Sox haven't actually exchanged numbers because then it becomes obvious to the White Sox that their tepid interest is actually among the strongest. It tips the agent's hand toward the White Sox, who then realize they can be patient.
This is why the offseason is more fun than I think most people realize -- all the gamesmanship and intrigue and doublespeak that's happening just under the surface.