It's a bit complicated:

1. Sapling can refer to either the system as a whole, or just the client people use (`sl`, forked from `hg`).

2. EdenFS is the virtualized FUSE filesystem (closely modeled after SrcFS at Google).

3. Mononoke is the distributed system that implements the backend server storage + the API they describe for checkout-less writes.

All three come together to form the core of the Meta source control experience. From what I can tell, Anthropic built its own version of 1 and copied 2 and 3, plus also built out a code review tool separately.