They need a simple GUI on top of rclone. The madlads of rclone fucking reversed engineered the drive APIs in record time. Now imagine if they were to tosh some money into that project, and then could focus only in GUI.
They didn’t have to reverse engineer the drive API. Proton created an open source library to use their API, which was forked to integrate with Proton-API-Bridge, so that apps could easily use it.
There’s no api, or there was, when rclone implement it.
This backend uses the Proton-API-Bridge, which is based on go-proton-api, a fork of the official repo.
According to that page there was an official API library, provided by Proton. They forked it and added features, it didn’t need to be reverse engineered.