I've built some ALU's and prototypes of CPU's, here is some grandstanding of my TMS1000 clone: http://www.youtube.com/watch?v=t82p-Ql8Djo The thing is though, while I am an older guy, I went from zero knowledge to this in under 6 months in minecraft. Binary logic is just not that hard. Its just really really repetitive.
After the hundreds of test projects I built, I have a good judge on the amount of logic you need and I have to say, the kid knows his shit. He is basically using 3 shift registers and a shift ALU. That is, he is only calculating one digit at a time with bits hold carry/overflow flags. Notice how he said it was a 6 bit BCD adder? That is JUST enough bits to calculate 9x9 with one bit over for carry. This is why those cheap 99 cent solar calculators are so cheap, it has about the same kind of logic in them. If you build your logic around needing to shift, everything is smaller and easier. It might be slower, but it properly takes longer to cycle the display than to multiply a number. Also, its so small is because he doesn't have a big ass ram bank. 9/10 cpu projects usually are big because of even a bank of 20 registers.
On the mine craft side of things, he uses pressure plates as they look a bit nicer and generally its an easier interface to wire. There is this is/was a big design stuff in the forms about dot matrix displays so I figure he put in his own display stuff there. The only thing I am not impressed with is the "graphing" part of his calculator. Lets face it, the entire core of the thing is a 6 BCD adder with shift logic. This thing is WAY to small and far to limited memory for it to have nothing but static formulas. Also, why didn't he put in a simple clock to just "draw" the grid instead of pressing a button?
But to be honest this is WAY better than that stupid HACK alu. I AM impressed with the original guy who made it, just not all the 100's of clones people have made of it. Very few people have made a 100% working cpu. Anyone can build an ALU, no one builds the state/decoding logic:P
Look at his older caculator, you can see how small the basic logic is there and how much you need for the display:P http://www.youtube.com/watch?v=aSC_YXuONZg