What bothers me even more is the precision of time they can keep the data: 13.8 billion years. Not 13.7, not 13.9 - no: they figured out 13.8 exactly. That's fake precision and remarkably close to the current age of the universe...
Turns out the biggest decay factor is nanograting and the biggest contributing physical quantity is temperature. They plotted what decay would look like on an Arrhenius plot. They both computed estimates and measured to confirm: the measurements are quite accurate. The specific claim is that they computed that at a specific temperature (462K) the medium will last 13.8Gy: their point is that it would last as long as the age of the universe at that temperature. They could've picked any other point on the time/temp scale, like "here's how long it would last at room temperature".
They did not, however, simply make up a number with no justification, let alone commit straight-up academic fraud.
The actual paper is here: https://www.researchgate.net/publication/297892219_Eternal_5...