This project was started originally at Stypi where we needed an editor with a better API than just getting/setting the text to support coauthoring. We didn't find any good enough and because it was core to our product, we started to build our own.
Stypi was acquired by Salesforce in 2012 but we continued to work on Quill because we felt it was a missing piece of the web. We're now working for GoInstant (also acquired by Salesforce in 2012), who are also working on some very cool stuff.
Quill was originally called Scribe but then another editor released a bit ago with the same name so we changed it to Quill.