Had those colonists known what they were looking at when they found the remnant, or did they just know there was something freaky about it and that it was probably dangerous? The Adamantine colonists had stored their heavy equipment down here, after the supplies stopped coming and they hadn’t needed the assembler anymore, but had wanted to keep it safe just in case the abandonment was temporary. This shaft hadn’t been on the schematic of the surface dock, so this was probably under the other structure, the complex with the weird ribs that the alien remnant-contaminated Pre-CR colonists might have compulsively constructed before they all killed each other or melted or whatever.