I was aware of this kind of structure when I coined 'prolly tree'. It's the same thing bup was doing, which I referenced in our design docs:

https://github.com/attic-labs/noms/blob/master/doc/intro.md#...

The reason I thought a new name was warranted is that a prolly tree stores structured data (a sorted set of k/v pairs, like a b-tree), not blob data. And it has the same interface and utility as a b-tree.

Is it a huge difference? No. A pretty minor adaptation of an existing idea. But still different enough to warrant a different name IMO.