Hello, I am a bit late to replay here, but checked a little witht he user. Guest registration works fine, it is the actuall onboard enrollment that doesn't work. I guess it's semantics, but the OS stated on the error message is a little bit wrong. The Lumia 950XL is actually running Windows 10 Mobile, not windows phone 8.1 as the web page says.
I have no doubt this is falling under the same currently not supported umbrella though and I will inquire our Aruba contact regarding the matter.