I was in no way trying to undercut Apple's efforts, just stating what's written in their documentation [1], i.e they're currently only performing tracking with VIO as compared with Google's [2].
"Almost by default a robust VO system using some kind of sensor fusion does a form of relocalization but rarely loop closure." This is not true, Visual(Inertial) Odometry systems typically estimate "odometry" i.e frame to frame motion tracking hence the name, such as in [3] and [4], whereas SLAM systems, such as [5], store a map of features, and their descriptors to relocalize which isn't necessarily a typical feature of pure VO systems.
[1] https://developer.apple.com/arkit
[2] https://developers.google.com/tango/developer-overview
[3] https://github.com/uzh-rpg/rpg_svo