People do fail to realize things that happen recently and what happens on new servers.
Recently, graphics bugging happens a lot. Even in Ares a couple weeks ago if I was being chased by a bunch of people, and getting hit, all you'd have to do is run backwards for a split second, then run forward again, and they'd run into your char when he wasn't really there, making you out of range. This can be done if you are running towards the right side of a wall, approach it, and suddenly switch to the left side of a wall.
Also on new servers, AC and hp are quite low. HP pot and minor heal usually can outheal most damage on a rogue. I'm not saying he doesn't cheat, but its quite possible he just used an hp pot + minor heal. Sins get me low in health pretty quick, but because everything past thrust hits for such low damage, I can usually pot back up to near full health before the skills regen.