No, you misunderstand: he mentions a common optimization where before you run your diffing algorithm, you find the common line suffix/prefix for the file, and how that will improve performance (if you have a compact 5-line diff in a 10,000 line file, it's unnecessary to run the diffing algorithm over the whole thing). His point was that this suffix/prefix finding thing was surprisingly slow totally apart from the actual diffing.

I was talking about that part, how hashing there is unnecessary. As I mentioned at the end of my comment, for the actual diffing algorithm, it's fine to hash away.