I wasn't able to catch Stark's segment on Baseball Tonight just now, but none of what I've read about it suggests he's saying anything I'm able to make sense of. Basically, Stark's idea, which he repeats here, is this:
Well, that's not much of a measure of seriousness, is it? Upton has a lot more value than Marte does. If the Pirates don't give up Cole or Taillon, it's hard to see how they're going to make this deal, unless they're going to make a bizarre, franchise-changing trade involving Pedro Alvarez or something. There are a few scenarios in which they could acquire Upton without giving up Cole or Taillon, but their options are pretty limited. If anything, the fact that they're unwilling to give up Taillon suggests they're not that serious. You can argue that's actually a good thing, of course. But Stark's report doesn't make a ton of sense to me.
UPDATE: The Pirates "have a scout watching Justin Upton in Chicago," Jon Paul Morosi reports.