I can confirm that the issue is 802.11ac and not BT 4.0 on my early 2013 Retina MBP -- which has BT 4.0 and 802.11 a/b/g/n.
Further, this Apple support kb, https://support.apple.com/kb/PH25156?viewlocale=en_US&locale=en_US, reinforces the wifi dependency in the following sentence:
Does anyone know if substituting a replacement 802.11ac wifi card can enable this feature?
Thanks for the info, I've been contemplating buying a Bluetooth Dongle for my Mac Pro, but now I may look into a replacement combo card instead. Too bad it's a lot more expensive than a $12 dongle.