Thanks for the question. I needed a term that would fit the README without sounding too extravagant either. Here is the explanation from fisher(7) [https://github.com/fisherman/fisherman/blob/master/man/man7/...]

Basically, a flat tree takes advantage of fish's lack of private scope (it bundles everything into the same "public" scope).

Fisherman then only needs to "load" the one single path where plugins/snippets/prompts/etc are transferred after being downloaded to the cache.

This is faster than, loading a path hierarchy per plugin and also means that performance is not affected based in the number of plugins installed.

Probably like the new npm 3 dependency directories. If A and B require a common version of C then the hierarchy is totally flat (A B C at the root). If they require incompatible versions of C then there will be 2 C, one nested inside A and one inside B