“Pointing solemnly to the plumbless mysteries of heaven” gives me an image of the towers accentuating the endless awe-inspiring capability of the night sky - but John’s the only one who can see that, as the “civilized” characters are too caught up in their hedonistic lives and soma trips to look up. I don’t think “fingers” needs to be read into too deeply, it’s just to go along with the pointing out metaphor.