Those links seemed to be about sticking rocks into/around fires.
I think boiling is different: the temp doesn't go as high as it would sitting in a fire. The outside of the rock is going to heat/boil before the inside does (probably a miniscule difference unless you are dealing with really big rocks). Also, water trapped in a cavity would be under pressure as it's heated, and this pressure would effectively drive up the boiling temp for the bit of liquid in that cavity. I'm wondering if it's something similar to why boiled eggs don't explode, but microwaved ones will?
Anyways, yeah, don't stick rocks in the oven, or on fires unless you know what you are doing (granite, basalt probably fine, sandstone, shale, slate, limestone - not so much).
and I don't see much point to boiling them anyways, but I don't think there is really any risk to it.
Back to topic, cool find, it does sound like maybe they are excavating or something. Hope some of you Manhattan folk can make some use of it.