AMD has been very clear that they don't say anything about their semi-custom customers chip before the customer has disclosed it itself, custom or pure stock AMD design.Yeah, they don't disclose technology that the partner (or themselves) didn't want disclosed yet. "based on RDNA" or "next gen RDNA" was the most they could say until RDNA 2 was officially unveiled.
Anything custom cannot be disclosed by AMD first, so it's a reasonable angle to argue it might not be amd's implementation, but it could also be just a slightly modified version of amd's implementation.