I suppose it does mean that ... but my mind rejected the idea of the girls making the truth softer. I simply don't like the phrasing, especially from someone like Oates, but perhaps I should expect it from a creative writer.
I can see hard truths, the truths learned at the forge, in the shackles, in poverty. But I don't imagine the opposite of hard truth to be soft truth, although I might be inclined to go along with "the soft truth of maternal love" if I read it in a passage.
But I don't see the girls sniggering at what they consider to be the politician's impossibly upbeat message that they can achieve anything as softening any truth. He has his truth, they have theirs ... and their jest doesn't soften or lessen any universal truth, not that one is being expressed anywhere than I can see.
Ah, well, that's just me. And perhaps it makes sense in the larger context.