The amount of respawn is affected by the difficulty level you choose, so it's possible he was playing on a different difficulty.
I decided to replay both the original SS1 (well, SS1EE but AFAIK they're the same) until i unlock the elevator and play the demo again. Originally i played the demo at the default difficulty setting (2/3) but now i played it at maximum difficulty for combat (i left the cyberspace and puzzles at 2/3, shouldn't make any difference). I also played the original SS1 at the default setting (3/4), so technically SS1 should be at a lower difficulty.
While there were a few more monsters in the level, they still only respawned once and i went multiple times over the same areas (especially the healing pod area). By the time i had unlocked the elevator everything was dead and nothing to shoot in sight - which was a bit disappointing since i had just picked up the magnum and wanted to try it out :-P. On the other hand in SS1 there were respawning monsters all the time, almost every time i went back to the healing pod area there were new monsters to the point that i collected 52 med patches just from the monsters i killed (and not all of them have those, many had just empty cans). At one point i even had killed every monster in a room only to turn around (literally just turned around, i didn't move) and when i turned back there were three new monsters in front of me :-P.
Regardless, now that i played both one after the other i can easily say that aside from respawning it is a faithful remake and an improvement over the original (I even changed my mind about the sobbing in a couple of recordings since if you think about it those people aren't some hardened marines, but scientists and other civilians... and some of the recordings in the original do not sound that great TBH).