Considering the original claim was "zero indication that the mic is hot" and now it's "zero indication that the mic is hot if the screen is off", I'd say that the goal post has moved considerably.
But if you want to know if Facebook is listening to you through the iPhone microphone, you should probably look at the screen for the indicator. iOS apps can't start recording on their own in the background, there's no API for that. If they are listening to you, they'd have to start the audio session in the foreground, which would allow you to see the indicator.
I wrote the original "Wife swipes open the phone" comment, so that's the context you seem to be missing. Sure you can see a little dot on your phone when YOU run some experiment today and look for it, but was that indicator available in the exact situation where the targeted ad was displayed? No.
Also, this incident happened in the past and we know there have been dramatic API changes on both Apple and Facebook products. The limits of the API today don't reflect the capabilities that were available to developers in the past. I doubt Facebook is hacking the App Store process to use hidden APIs. It was probably just available in the past and my wife granted the facebook app complete access to the mic, so they took what they wanted.
I'd make sure to disable that permission today too, just in case.
One last thing is I just opened my iPhone again and hit record. I honestly didn't see the tiny orange pixel at the top of my phone until you pointed it out. I was basically looking for the green video indicator light to show. So I'm technically wrong about NO indication, you're welcome.
Considering the original claim was "zero indication that the mic is hot" and now it's "zero indication that the mic is hot if the screen is off", I'd say that the goal post has moved considerably.
But if you want to know if Facebook is listening to you through the iPhone microphone, you should probably look at the screen for the indicator. iOS apps can't start recording on their own in the background, there's no API for that. If they are listening to you, they'd have to start the audio session in the foreground, which would allow you to see the indicator.
https://developer.apple.com/forums/thread/65604
https://stackoverflow.com/questions/70562929/how-to-start-au...
(Unless you believe that Facebook is using some kind of a private system API for this and is passing through the App Store checks)