The 13.8Gy claim is made by estimating, calculating and then confirming by experiment how quickly wear and tear works (the primary mechanism is nanograting, and the primary contributing physical quantity is temperature). It then specifically grabs 13.8Gy as an example data point because that's the age of the Universe, but they could've picked any other point on a time/temperature scale, like, say, "room temperature".

Are you suggesting they pulled this out of their backside? The research very much supports that number.