I agree, it was a far-fetched kind of moment. But it wasn't different from a similar moment in Halo 2, really.
I don't think we need to spoiler code Halo 2: That game came out nearly a decade ago :P
And really? At least there was 5 seconds of cutscene showing him shooting away from the carrier. Through the magic of editing that could have well been 5 minutes, it wasn't anything like the end of 4. 5 seconds of high acceleration in Zero G puts him at what, 100m away at least? He's in a pressure suit which has comparable armor to a spaceship. That's plausible. What happens at the end of 4 really isn't bar some deus ex machina we've yet find out about.