technically grenades should still fall to the floor albeit much slower.... the same low gravity that makes you fall slower would pull down on the grenades, although it would take them significantly longer to hit the ground.
yeah the pfhor ship levels are probably the best-designed levels of any game ive ever played, and ive played quite a few games. compared to todays standards the M1 engine is of course old and outdated but the important thing is that Bungie used the M1 engine to its full extent... i have never seen anythign in Quake 3 or Unreal Tournament that uses the engine to its full extent the same way that Marathon did.
yes i know that Q3 and UT are very very very different from each other and from M1, im just making a point