That’s relevant. It means that it’s not just the display, but the entire machine.
Unfortunately, this sounds a lot like what I encountered with my Apple hardware from around the same age as yours:
The issue is that those Xeon CPUs in your hardware aren’t 1-to-1 with what you’d find in other PC hardware. They were made specially for Apple, and could potentially have some key functions missing (Apple did this a lot with their hardware), that the Qubes ISO is expecting to find, hence the errors.
Give me a second. I’m checking up on how to launch the Qubes ISO in text-mode only…