Yeah, I noticed Mansell's absence too, but then looking at the soundtrack he's only credited for two tracks; 'Leaving Earth' & 'An End, Once and For All'. Makes sense though as those two basically bookend the game, but the latter has a co-credit with Hulick, so...yeah. He's more of a guest contributor really.
Yeah, it really irritated me that Mansell only did two tracks, given how much they hyped his participation (Crysis 2 did a similar thing with Hans Zimmer). It's even worse than what Crysis 2 pulled because Mansell's tracks are pretty much the only ones that have any sort of staying power, although Das Malefitz is a pretty catchy track.