Prohm could know that Lindell is coming back and not tell us, but rather wants Lindell to be out there as much as possible. Prohm knew about Monte coming back a good ways ahead of the announcement by the University two years ago. Leading up to that, there was a lot of skepticism about whether he would be coming back or not, and Prohm even played it off like he wasn't sure how it would go.
I think Prohm's playing this the same way. It can only be a positive for Lindell and Iowa State to have his name in so many teams' workouts and all that Jazz. Even if you know he's coming back theres no reason to tell anyone. Keep his name in NBA teams' circles and hopefully develop some buzz going into next season,