This is a joke, of course, referring to the chief engineer's next line of dialogue. But there is another meaning behind it. Mountains in Nostromo symbolize the pure Ideal, the root dream of attainable human happiness that lies above all the varying idealisms. "Faith," or religion, can move mountains because it deals directly with this higher level of human aspiration. Material progress, on the other hand, is merely a means. It can refer outwards only as far as the inanimate, recalcitrant physical world (note the "basalt walls" in the next sentence, around the "arena" of human action). Material progress makes no attempt at grander explanations to explain the mystery of the universe; indeed, it pretends to not be a belief system at all, but to be founded on fact. The "mountains" that it cannot move, but religion can, are those of the deepest human questions and the highest human ideals.

Compare Dr. Monygham's denunciation of material progress as being "founded on expediency" ; i.e., it has nothing to say about what mankind should be expedient in the service of.