The FAQ page (via WindowsArea.de) notes that the HP Elite x3 will get some new additions with the Anniversary Update, such as the Windows Information Protection security system, along with some improvements to the phone's Continuum and other features. It should be pointed out that the HP Elite x3 can also connect to the Windows Insider Program, so people who get the phone before Sept. 13 can connect to the Release Preview ring and snag the Anniversary Update, if they want.
Obviously, release schedules for software updates can, and usually do, change, so don't take the Sept. 13 date as being written in stone. The HP Elite x3 has started shipping in some parts of Europe. The U.S. can expect shipments to begin in early to mid-September.