Interesting. Does swiping left-to-right bring you back?
Update: I had a chance to test, and swiping horizontally from left to right performs the back gesture and returns the user back to Discourse. You can also swipe from up to down, that’s a gesture to dismiss the webview, it will bring you back to the site list of the app.
For the record, this issue doesn’t affect meta, because meta has CDN for assets and the Download link is treated like an external link by the app, that is it sends that link to Safari.