I forgot I had it installed on my phone, but clicking the link opened, to my surprise, the Simply Piano app. Why you'd associate your app with that domain for legitimate purposes is a bit of a mystery to me.
This shouldn't be possible on Android. Google requires apps capturing http/s deeplinks verify ownership of the URI.[1] They can accept any other schema declared in their manifest otherwise. Example code would seem most plausible, but I can't see how it would work.
