Reminds me of the anonymized AOL search dump from back in the day[1]. Needless to say, the dump was a trove of personally identifiable information despite being "anonymized".
My vague recollection is Apple would actually take the Siri request and route it to Bing for you, so Bing doesn't know your IP, and they would attach a unique identifier that they'd rotate every 15-or-so minutes, so that way Bing can correlate multiple requests done back-to-back but loses any correlation with requests outside that 15-minute window.
[1] https://en.m.wikipedia.org/wiki/AOL_search_data_leak