I feel like this sentence is misleading; it implies that the 'payload' Google's interested in is personal user data, as if Google has a team of engineers in its basements looking through the cat photos and credit card numbers it captured in Wi-fi packets.

"Payload data," in this case, is most likely fragmented packets for which it can glean metrics on how to make the process more expedient in the future.

Of course, I can't say this with 100% certainty, as I'm not involved, but the entire story seems geared to make Google's capture of radio-broadcast data packets seem far more sinister than it really is.