Given android's upward trending market share dominance, android-first will probably be the default in the next year, with android-only becoming the norm in 3-5 years. As the article points out - you've got to follow the numbers. At that point, iOS will be treated similarly to mac os - there will be groups of people who love and swear by it, but while they'll be rich, they won't be the mainstream market.
Android-first? I'm pretty sure Apple would never allow an app like Cover since it would rely on all sorts of Private APIs tp take over the home and lock screen. Cover is 'Android-only' not 'Android-first'...
The thesis of the article is that Cover makes Android so compelling that other app developers go for Android first because they'll get more usage via Cover surfacing their app better post-installation.