Source : https://www.techquila.co.in/intel-kaby-lake-g-polaris-gpu-vega/
Intel Kaby Lake-G, (the Intel Haydes Canyon NUC specifically) might not have Vega graphics in it as advertised.
However, SiSoftware Sandra lists the Intel Haydes Canyon NUC to have the integrated GPU as Radeon RX Vega M GH.
To summarize, the factors that help the argument that we might be getting the older Polaris instead of Vega are:
DX12.0 instead of DX12.1.
No Rapid Packed Maths support introduced with Vega.
Uncertainty whether these can be enabled via a software patch.
Giving Intel/AMD the benefit of the doubt, the graphics solution packed with Intel Core i7-8809G in the NUC was custom designed for Intel, so accordingly, architecture modification to a certain extent is certainly possible. However, one thing can be said for sure:
No comment from Intel/AMD, as of writing this post.