Is Simple Note open source?

I'm searching for a good open source note taking app and I don't really like standard notes or Joplin. So I want to test simplenote but at first it requires an account to login and secondly although you have a repository on Github I don't see you mentioning if it's open source or not.

The issue names no files, tests, or entry points. Start by checking the repository's licensing and account requirements, then determine whether the project documentation clearly states Simplenote's open-source status and login requirement; done requires a maintainer-confirmed clarification.

Written by the indexing model from the issue text.