Fusion Drive is just flashcache with some sort of adaptive caching algo. There's zero reason to care about what the type of a file is. The reason they showed it like that in the keynote was because normal people don't know what a block device is.
Instead, this appears to be an implementation of so-called "tiered storage". This is novel in that is the first time such an approach has been applied to consumer grade solutions. Usually, one might find tiered storage as part of an extremely expensive solution that EMC might give you as part of an elaborate storage system.