Those indicators aren't trustworthy. If your phone is configured to respond to "Hey Google", it will do so even when you say it while that indicator isn't lit up.
It is always a question of trust, but after more than a decade of phones they are pretty determined to earn trust. I have not seen any trustworthy reports of a secret always on microphone. Just the occasional recording made after a misheard trigger word or button which is an issue being further secured year by year.
Amazon on the other hand…
It can easily explain how people with iPhones are being eavesdropped on: "those indicators aren't trustworthy". According to this theory, the microphone in an iPhone is active despite the indicator not being displayed. That's identical to the Android theory.
To extend the theory to Facebook, you'd need to assume one of these:
(1) The unreliability of the indicator on Android is a matter of incompetence on the part of Google, and Facebook is exploiting it.
(2) Google has a privileged stealth-microphone-access permission, and Facebook has negotiated for access to it.
A: You know phone microphones aren't secretly taping us because indicators display when they're active.
B: Those indicators aren't reliable.
A: That can't explain how iPhone users would get taped.
Where did that come from?