They seem to speculate that the cells have been in a low-energy state where they either divide very slowly or spend all of their energy repairing broken molecul

They seem to speculate that the cells have been in a low-energy state where they either divide very slowly or spend all of their energy repairing broken molecules, which would presumably include their own DNA. My own (baseless) speculation: Maybe the population was originally much larger, and the surviving cells have been maintaining the energy to live by slowly cannibalizing each other over millions of years?