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.
You'd need the right info at URLs like:
https://example.com/apple-app-site-association
https://example.com/.well-known/apple-app-site-association
https://example.com/.well-known/assetlinks.json (Android)
(which obviously don't exist for this domain)