Not sure about eating xenos, but why not. This would be the "dark secret" they share withe the DAs.
But I'm still more inclined towards sallies.
Indeed, to detail a bit more about their beliefs, I suggest: They are each engaged on a quest to ascend to godhood with the Emperor. for them, the world is not "everything has a soul, wolves are our equals...", but quite the opposite. It's all about evolution. primordial forces becoming beasts becoming men becoming marines becoming gods. They are regarded as under-evolved, dammit. By recycling their environment, learning to master it (sculpting stone, carving bones, killing beasts, eating men) they learn to master themselves (hypno-conditionning) and slowly become more than what they were previously. and there we go, 2001 reference, with cannibalism.
The Megalith, being incredibly ancient, durable and unchanging, is to them some sort of anti-emperor that they chose to inhabit, to master and to use, but still revere as a material equivalent of godhood.