It is true, and what you quoted does not contradict this.

https://extensionworkshop.com/documentation/publish/signing-...

You can temporarily install extensions in about:debugging, but everything permanent needs to be signed.

> Add-ons need to be signed before they can be installed into release and beta versions of Firefox. This signing process takes place through addons.mozilla.org (AMO), whether you choose to distribute your add-on through AMO or to do it yourself.