Students have been using graphing calculators to play games since at least the TI-82. It was policy at my high school that the math teachers would take them away and wipe the memory if you seemed to be goofing off with them.
Of course on the TI-82 it was always simple arcade games like snake and submarine hunt, not Doom... but the modern generations of graphing calculator are pretty damn powerful and are optimized to do the same sort of calculations that a graphics card does, so it's not surprising to see an FPS ported to one.
What is surprising is that TI still gets away with a puke-green 1980s LCD in a calculator that costs as much as a netbook. The physical parts in those machines combined and assembled aren't worth more than $30, they could at least give their customers proper black and white displays.