I’m a borg guy. I’d never heard of kopia. This is from their docs though:

Each snapshot is always incremental. This means that all data is uploaded once to the repository based on file content, and a file is only re-uploaded to the repository if the file is modified. Kopia uses file splitting based on rolling hash, which allows efficient handling of changes to very large files: any file that gets modified is efficiently snapshotted by only uploading the changed parts and not the entire file.

So looks like they do append only.