It will cache credentials for a short time so you can still access some of your passwords. It will not let you add new credentials. It's like a web browser working in offline mode for a period of time. It is a cloud-based password manager with a closed-source server backend.

"A walled garden is a garden enclosed by high walls, especially when this is done for horticultural rather than security purposes, although originally all gardens may have been enclosed for protection from animal or human intruders."

Agreed proton isn't this

"A closed platform, walled garden, or closed ecosystem[1][2] is a software system wherein the carrier or service provider has control over applications, content, and/or media, and restricts convenient access to non-approved applicants or content. "

Try using thunderbird and id argue proton is this