tl;dr: If you install and fully trust a root CA on your client device, of course your TLS traffic can be MITMed. edit: the problem, obviously, is that this app tricked the non-technical people into installing/trusting the root CA for malicious purposes. Clearly this was malware.
That's great for someone reading this forum to be aware of, but moms have no idea what any of the words you just wrote means. So if they were told they get a coupon for installing or some other bit of ridiculous things malware devs use, and yes I'm calling FB software malware. All of if it. Messenger, FB.app, everything. If it's from Meta, it's malicious.
As is pointed out in the article, I would presume that Google saw the threat from allowing an app to install and trust a root CA as well, and removed the ability for a "one click" install of a root CA:
"KeyChain.createInstallIntent() stopped working in Android 7 (Nougat). A user would have to manually install the certificate. It would no longer be possible to have Facebook's CA cert installed directly in the app."