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.