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.
This is an rclone backend for Proton Drive which supports the file transfer features of Proton Drive using the same client-side encryption.
Due to the fact that Proton Drive doesn’t publish its API documentation, this backend is implemented with best efforts by reading the open-sourced client source code and observing the Proton Drive traffic in the browser.
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.
https://github.com/rclone/rclone/blob/master/docs/content/protondrive.md#-icon-fa-fa-folder--proton-drive
There’s no api, or there was, when rclone implement it.
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.
According you the page, no, you’re still wrong.