Since when does Facebook lie about notifications in order to encourage a behavior of downloading messenger, if people don't know about their mobile browser's "view as desktop", allegedly? I mean, it is a NEW notification indicator after all, not about a message I got a week ago and read then (that wouldn't actually notify me of anything, so the lie would be about it being a notification at all).
Both Facebook and Instagram have been doing fake notifications for a couple years; for me the end effect is to ignore all red icons or notifications, period. I'm sure someone thought it was a great engagement pattern, but it is also a major trust killer.