I’ve always stood by the position it’s totally possible to snoop audio and match it to a bloom filter or on device least of keywords for ads. Siri is always on so your mic can be always listening and have no impact on the battery life.
In modern mobile OSs it should be clear that your mic is either on or off (to apps) and we don’t see the mic on all the time as would be needed for this. Maybe there’s a hack, but at the scale and being used for commercial services I think someone would have noticed.