Perhaps better worded as “still missing couple of features that chrome requires.”
Whilst the manifest.json and the meta tag are part of what is required there is still a matter of following before the “add to homescreen” will be seen:
- Site must be delivered over HTTPS (hopefully Let’s Encrypt support will get this one for everybody).
- Must have a service worker registered on your site (I didn’t spot one in Discourse’s code).
- The user has visited your site twice over two separate days during the course of two weeks.
(this list might change in the future).
EDIT:
Although I don’t see the service worker requirement on this page:
https://developer.chrome.com/multidevice/android/installtohomescreen
But there is a mention of AppCache.