I don't think there's anything to be gained by switching to VHDL or Verilog. Their custom HDL is a fine HDL for the uses of the course. There's so much commonality that they're effectively interchangeable syntactically. If the idea was to use VHDL or Verilog such that one could actually make the CPU on silicon or an FPGA, then that opens up a whole can of worms that is well beyond the scope of the course. I do think it would be awesome if they had a sequel that did this, but I think it doesn't make sense in the context of the existing course.