I'm not an expert on the iOS security model so I'm happy to be corrected about this, but as far as I understand this (enabling the mic without the alert banner

I'm not an expert on the iOS security model so I'm happy to be corrected about this, but as far as I understand this (enabling the mic without the alert banner or fetching data from wherever Siri might cache requests) would mean that either Apple is secretly colluding with Facebook or that Facebook is using an iOS exploit and sneaking it past app review. Thats an extraordinary claim that needs more then yet another anecdote and a vague technical expert to back it up, especially since it would not be unreasonably hard to provide some technical proof for this.