I can see why people are quick to think this but I don’t see any compelling evidence this is the case, and as others have pointed out it would be impractical for them to do so.

More likely they use it for consumer lock-in and to collect data through its api endpoints. Collecting media activity and smart home device information is valuable enough on its own, before even approaching the value of collecting recorded audio.

They can already intuit consumer habits/word of mouth exposure from other associated data with your online activity. After locking down all my other privacy, the ads I get are far less relevant to me, even though I have a number of smart listening devices in my home