since u seem to have knowledge about the subject (which i lack), why cant the system just revert the commands? simplefied:
command 1
command 2
command 3
rewind:
command 3
command 2
command 1
Game engine doesn't work like that. You can fire a gun, but you can't "unfire" a gun.
In some ways, game replays resemble video compression algorithms. In video compression algorithms, they store transformation vectors (movement, rotation, size change), and then "play" those much like the commands in a game replay. In order to have rewind/skip ahead working in compressed video, the video file will have periodic "key frames", where the full video frame is stored. This would be equivalent to storing the full, current state of the game, rather than just commands. When skipping to a different portion of the video, it finds the most recent key frame, performs all the transformations from that point until the point you asked it to skip to.
I'm guessing coh2 doesn't have key frames (aside from the beginning of the game being in effect the staring key frame), so skipping is impossible without replaying from everything from the beginning. Adding key frames is a large undertaking, hence it's not there. It's not a trivial task.