MacLaren is a Traveler
Just like all the others, a person from the future came back and occupied his body at the exact second of his "historical time of death." We see this happen to each of the others in the team as well, but since their deaths were more immediate, we see the take over happen when they actually die.
Note that because MacLaren originally died from falling down an elevator shaft, he wouldn't have died until he hit the bottom, which means there was some "lag time" in between when the others intervened and stopped him from falling, and when he actually would have died. This is why the traveler arrives a few seconds after they "save him."
The purpose of the rule about not saving people is that if they save someone who should have died, that would alter the timeline. But saving someone like MacLaren because they intend to possess his body isn't really saving him, it's just keeping his body intact for the incoming traveler.