This is my conclusion too.
Regarding alternative browsers, the "explainer" says that all browsers on a platform would be able to be attested. But this doesn't really matter, because support for arbitrary browser extensions nullifies all the trust requirements identified at the start of the document.
In other words, the moment a browser hits the Play store which both supports WEI and allows arbitrary extensions, sites relying on WEI will start ignoring it if it's sent from browsers other than Chrome.