Is there a good explanation of how that isn’t just measuring the expansion and contraction of a ruler with itself? Don’t we know the reactor is 2 billion years old because of radio dating?
The isotopes produced during the natural nuclear reactor 2 billion years ago were produced in certain ratios because of the relative sizes of their nuclear cross sections, which depend on the fine structure constant.
The isotopes used in radio dating are produced by spontaneous transmutation over time, which is governed by entirely different processes.
Which is why a synthetic clock is needed here. That will have a known inception date and the changes if any can be compared.
The problem with both is they're not exactly fully closed systems anyway so there will be some margin of error ever with the length of the operation.
And during the test, we might just find out something completely unaccounted for in current physics... That isn't a universal constant related at all.