I wonder if there’s some kind of side channel leaking the ad targeting data, for instance if you type something into WhatsApp and they add words that you type into a user-specific autocomplete dictionary (or your lexicographical profile or whatever other aggregate they keep for various business purposes). Then they use those aggregates to target users based on the words they use in conversation without “decrypting their messages” so to speak. Depending on what phone you use, any custom keyboards, etc there may be other ways for typed text to leak.