Whether or not that is true is based on a core assumption, and depends on your perspective. It assumes that by not expanding the capabilities of BES to support Android and iOS that RIM could somehow stop the exodus away from its BlackBerry mobile devices. That's unlikely.
As for perspective, BlackBerry Mobile Fusion may accelerate migration off of BlackBerry mobile devices, but it establishes RIM as a leading player for multi-platform mobile device management at the same time. BlackBerry as a mobile OS, and BlackBerry devices may continue to decline into obscurity or oblivion, but RIM can live on by shifting its strategy to that of a software company capable of providing the multi-platform mobile device management and security infrastructure customers need.
If Dell can brand itself as "not really a PC company", then RIM may just be able to transform itself into "not really a mobile device company". BlackBerry Mobile Fusion may be the thing that saves RIM from itself and removes some of the tarnish from the once proud brand.