Its all about firmware! Intel CPU’s load some of their micro-code from the systems ROM (EFI in our case here).
Apple never offered the T9300 or T9500 CPU’s so the micro code within the EFI is just not there! Sadly there are a few people who assume something works without doing it or understanding the limitations Apple imposes. I would make sure you have the newest EFI this series used as maybe it snuck in! I haven’t done this upgrade so I just don’t know if the issue is just a versioning issue within the EFI.
The highest of the T series CPU’s Apple used was the T7700. Later on Apple did the X7900 as the Extreme and then jumped to the E series in the 24” models.
So you have proved anyone telling us this series can support the T9500 is full of it!