> The only relevant difference between Radar and WebKit Bugzilla is private vs. public.
No, this is not true. Radar is not just "a bug tracker" — it is a very powerful project management tool, custom-built over several decades for the way Apple works, with integrations into other internal tools and processes.
It should be obvious why Apple wants to track work being done on Webkit using the same tool it uses to track work being done on every other project at the company.
Last time I saw Radar, and its source code, which was admittedly over a decade ago, it was not a "project management tool" as such, although radars could of course be used in project management. It was indeed just a bug tracker, but yes, a very powerful one. At bottom it was a giant Oracle database, with several frontends.
bugs.webkit.org is literally a bug tracker.
The only relevant difference between Radar and WebKit Bugzilla is private vs. public.