I think they're lying. Apps can't access the microphone, on an OS level, without explicit permission from the user. Unpess Facebook found and used insane exploits in both Android and iOS, I'm calling bullshit. They're just bragging about something they can't possibly do.
Wouldn't want to be mean to Facebook users, but the vast majority of them probably has micophone access enabled for Messenger at least, if not Facebook.
At least on iOS, it takes it a step farther and tells you specifically when an app is accessing your location, microphone, camera, etc… It even delineates when it’s in the foreground or background. For instance, if I check my weather app, I get this symbol in the upper corner:
The circled arrow means it is actively accessing my location. And if I close the app, it gives me this instead:
The uncircled arrow means my location was accessed in the foreground recently. And if it happens entirely in the background, (like maybe Google has accessed my location to check travel time for an upcoming calendar event,) then the arrow will be an outline instead of being filled in.
The same basic rules apply for camera and mic access. If it accesses my mic, I get an orange dot. If it accesses my camera, I get a green dot.