Was this mentioned somewhere else? I didn't see it. Pretty big news, no other TB3-2 adapter allows this, its' all one way.
http://www.apple.com/shop/product/MMEL2/thunderbolt-3-usb-c-to-thunderbolt-2-adapter?fnode=4c
The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lets you connect Thunderbolt and Thunderbolt 2 devices — external hard drives and Thunderbolt displays, for example — to any of the Thunderbolt 3 (USB-C) ports on your MacBook Pro. As a bidirectional adapter, it can also connect new Thunderbolt 3 devices to a Mac with a Thunderbolt or Thunderbolt 2 port.*
* Requires macOS Sierra.
http://www.apple.com/shop/product/MMEL2/thunderbolt-3-usb-c-to-thunderbolt-2-adapter?fnode=4c
The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lets you connect Thunderbolt and Thunderbolt 2 devices — external hard drives and Thunderbolt displays, for example — to any of the Thunderbolt 3 (USB-C) ports on your MacBook Pro. As a bidirectional adapter, it can also connect new Thunderbolt 3 devices to a Mac with a Thunderbolt or Thunderbolt 2 port.*
* Requires macOS Sierra.