pxlkng apps obtained via Aurora come with the same libraries that use many APIs dependent on presence of Play Services and without them many simply don't work. Without authentication to Google they are likely assigned pseudoanonymous ID, correlating can be provided by other tracking libraries not dependent on Google Play. But generally speaking, tracking is diminished compared to infrastructure incorporating Play Services, for each app at a different level according to libraries used.
Never said it is equally secure but there is a userbase that is interested in this approach, especially if they look to get access to apps that don't require network in order to function.