I don't understand the drive to make every app look exactly like Apple's own apps. So now FB Messenger is going to have tiny, unrecognizable discs for avatars and stretchy, flat message bubbles that fade as you scroll? Seriously? Why??
The article only says they're testing a version that looks like the Messages app, whatever that means.
If you look at the main Facebook app, it still has square avatars, no crazy whitespace and messages don't fade out. I think the tab bar is very old-school iOS too. I don't like the iOS 7 core apps, but Facebook hasn't adopted any of the craziness.