I feel like I'm completely missing something there. The quarter-round proceeds as follows:
b ^= (a + d) <<< 7; c ^= (b + a) <<< 9; d ^= (c + b) <<< 13; a ^= (d + c) <<< 18;
So at each step, we're adding words a, b, c, d together. And at the end of 20 quarter-rounds, we add the current state matrix with the previous state.
What I don't see is how overflows are handled. If two words added exceed the value of a word, does it simply wrap around?