Writing in that style is not that hard, I do something similar if I need to make a prototype of something. The short variable and function names really save time as long as you remember what is what. Reading or debugging it even a week later is torture tho.

Writing the interpreter in an afternoon is impressive but most likely he had done something similar before and had most of the algorithm in his head before he even began.

You don't need to be superhuman to do that.