OK, the fellow in the Genius Bar may have been confused on two counts.
1) iMovie 11 is called Version 9 (go figure), while iMovie 09 (part of iLife 09) is called version 8. You are currently on iMovie 08, which is version 7.1.4.
2) To make it further complicated, iMovie 11 should run on Snow Leopard, but only up to version 9.0.4. If you have Snow Leopard, you cannot update to 9.0.5, so 9.0.4 is the last update for you. Once they added Retina Display support, it would no longer run on Snow Leopard.
So the version currently selling in the App Store will not run on Snow Leopard.
Once again, you can go on eBay or Amazon and get iLife 11 (or iLife 09).
Also, it would not hurt to contact Mac App Store support and see if they can sell you iMovie 11 in an earlier version. They may get this question a lot.