I can test but I don't think deep links work if apps are not installed first. It will likely go back and forth between the browser and the app (first to install and then deeplink to onboard). And if that is the case, then it doesn't make sense to be an Amethyst link, given that people can choose Nostur or Snort, etc, if they are not on Android.
Discussion
BTW, I am not saying that Amethyst won't have onboarding, on-boarding a friend requires some flexibility on the client choice.