Earlier quoted context omitted.
You can't use the app for anything except this one service. If you're not reviewing the service itself at the same time, what's the point of reviewing the app?
You're reviewing how good of a UX the app has for the underlying service, as I see it. Because it's very easy to imagine a great stock broker with an awful UX. By your logic all variations of Facebook Messenger (like Lite, etc.) should receive the same rating? Surely that doesn't make sense?
Yes I would expect Facebook Messenger app review to include the facebook messenger service experience.