NP 
I think the point for both you and the authors of the app is that there are many changes from CR 8.5 (which they use to create the reports) and CR 2011 (6 distinct versions worth of changes actually). And each version adds new functionality, new ways of doing things, etc., etc. For them to say that they completely support CR 2011 in the app is a pretty big stretch. E.g.; a simple report with a few fields and objects, graphs and text will work. But if you use things like dynamic parameters, graphic format formulas, etc., you're out of luck. CR 2011 is also keeping up with the different database clients. So if in CR 2011 you use a new db client (say for Oracle 11, or "MS SQL Server Native client 10", or Native MS Access 2010, etc.) the CR 9 runtime will have no idea what to do with that...
I'd go to the author, point them at the link to this Discussion and see if they can pipe in with any questions / comments they may have.
Oh, just in case you are wondering what version of CR to use with their app. Obviously, if they are using CR 9 runtime, the best version of CR to use to create your own reports would be CR 9. With care you could go to CR 10, perhaps CR 11.0.x, but I would not go higher. And I do not think CR 10 or 11 is available for purchase anymore.
Good luck,
- Ludek