I think it depends on where in the world the user is from.

Where I'm located, it doesn't seem like WhatsApp is used much, or at all. I don't think I've ever met anyone in my personal or professional lives who uses it.

So if I were building an app using these APIs, it probably wouldn't even occur to me to test it with WhatsApp. I'd test it with Messenger, sure, and SMS. Maybe even Hangouts, or whatever the newest Google chat app is.

I don't think it's farfetched to think that someone could have received one of these notices even though they've never used WhatsApp.