Its own CPU, and a PCIe endpoint? Sounds like a good persistent exploit vector. I wonder what tasks it will be used to accelerate.
Mostly it's just fun seeing more and more co-processing devices arrive in phones as they get broken :)
I suspect they hope it will be the latter rather than the former. Design models at HQ, then distribute them to be trained on each user's data on their own phone. That means you need to use less bandwidth to transfer training data and models, which leaves more available to serve ads!