FUD. Apps cannot listen to the microphone without going through the OS first. I call FUD or share with me this magical OS-bypassing code. Compromising the OS at such a fundamental level on a recent Android version is almost certainly beyond their capabilities. I am more likely to believe the content that inspired this article is more aimed at investors and is blatantly making false claims, and that such claims from the privacy policy are generic disclaimers.
Further, have you ever tried to get an app to consistently run in the background on purpose? It's an enormous PITA when you actually want this to happen. Android apps do not typically run in the background at all unless they have again special permissions to bypass background restrictions. The OS strongly prefers to pause and eventually kill apps to save battery rather than permit background activities to occur unless they fall into specific categories and then only at specific times to optimize the battery usage.
If an app asks to run in the background all the time, bypass battery restrictions, and you grant it microphone access explicitly, the problem is no longer Android. The problem is the user being stupid by granting access against their own interests. And even then, it'll still trip the microphone indicator.