Your iPhone and iPad which use the Lightroom mobile application sync to your Lightroom full application on your desktop or laptop computer. You sync collections on your main computer to the Creative Cloud and then you can access them from the mobile devices.
I'm referring to the reverse workflow, in which my iPhone photos are sent to my computer. This requires that I set up a collection from within the mobile app. When I do so, I am given option to include only from certain albums on the phone.
Unfortunately I suspect this is an iOS issue and not something lightroom mobile can work around. I just didn't want my entire iPhone photo roll syncing to lightroom, just my favorites selected on the phone.