alt Hacker NewsIt would only be a backdoor if it's implemented as a backdoor.
The way Apple Health exchanges data with 3rd-party trackers (Fitbit, Garmin, etc.) is very well built and a good model of how other components in iOS could allow data exchange with very granular permissions.
Apple touts the "Private Cloud Compute". If they found a way to share your personal context to process on their cloud in a private and anonymized way, there is no reason the same process couldn't be used to handoff data to a 3rd party AI provider.
The technical problem is nothing like exchanging data with fitness trackers.
One of the issues here is that there are many people with strong opinions that don't understand the thing they have strong opinions about. Which is the normal state of human affairs.