Chrome published the WebExtensions API, not Mozilla. Whether or not it’s a standard is another question, you could call it a defacto standard, but Chrome definitely implemented the api that they published.
Mozilla as even stopped using the term WebExtensions, instead just going with Extensions [2]
The "spec" that most people refer to as WebExtensions, is BrowserExtensions orginally published by Microsoft via W3C [1]
[1] https://browserext.github.io/browserext/ [2] https://wiki.mozilla.org/Add-ons/Terminology#Background
Chrome published the WebExtensions API, not Mozilla. Whether or not it’s a standard is another question, you could call it a defacto standard, but Chrome definitely implemented the api that they published.