I bet he is talking about the difference that occurs when going from 480i to 480p.
That is that the normal TV interlacing adds a kind of anti-aliasing effect which most people take for granted until they move to HDTV.
His use of terminology is confusing. He is using "along the edges" not meaning the edge of the screen, but rather the edge of a line/region.
When he went to HDTV, he flipped the Xbox into progressive mode.
Now the jaggies, which were really there all along, look far worst.
The problem is that the games he is running do not all take advantage of the Xbox's one BIG feature, namely hardware anti-aliasing.
Games written specifically for the Xbox seem to do so, but others not.
Finally he is also not (yet) running any 720p game, else we would have heard how wonderful it looks.