This is disingenuous at best and incorrect at worst. The mute button on the Echo is just that, a button; it is not a switch. It is software-controlled and pushing it just sends a signal to the microcontroller to take some action. For instance, one action is to turn on the red indicator light; that's definitely not physically connected to the mute button.

Maybe another response of pushing the button is to disable the transistor used for the microphone, but it's more likely that it just sets a software flag for the algorithm to stop its processing of the microphone input signal. Regardless of which method it uses, the microcontroller could undoubtedly just decide to revert that and listen in, either disabling or not disabling the red light at the same time.

But I personally don't think it listens in when muted. I don't think it spies on us to target ads based on what we say around it. I'm not worried that the mic mute function doesn't work as intended.

But I fully understand that it is fully capable of it, technically speaking.