Syncthing will synchronize a full folder between an arbitrary number of machines, but you still have to access this folder one way or another.

Garage provides an HTTP API for your data, and handles internally the placement of this data among a set of possible replica nodes. But the data is not in the form of files on disk like the ones you upload to the API.

Syncthing is good for, e.g., synchronizing your documents or music collection between computers. Garage is good as a storage service for back-ups with e.g. Restic, for media files stored by a web application, for serving personal (static) web sites to the Internet. Of course, you can always run something like Nextcloud in front of Garage and get folder synchronization between computers somewhat like what you would get with Syncthing.

But to answer your question, yes, Garage only provides a S3-compatible API specifically.