I have heard numerous anecdotes from multiple people I know of them being served targeted ads shortly after saying something similar aloud with a device microphone nearby. You're right that there are a lot more ways than a microphone for ad targeting to work, but I wouldn't necessarily dismiss that guess as wrong.
Why not dismiss it? Like you say, the targeting could work in legal ways. That would be feasible and sensible. But what makes you doubt it is that you listened to a bunch of dweebs with superstitions?
