They're there before the tests though, and potentially more frequent around nuclear testing calendar days. The argument has never been "these only showed up after a nuclear test."

Two things here: radiation exposure could explain this, since there's a period after exposure and before developing where you can get radiation exposure.

Second, and perhaps more importantly, is that there's a detailed criticism of this line of research available, including evidence against the argument that these are more likely ±1 day of nuclear tests. See https://arxiv.org/pdf/2601.21946, and also https://arxiv.org/pdf/2402.00497 for a study of plate defect issues.

I think the current paper continuing this line of research should be read cautiously. I don't love discounting ideas out of hand, as these folks clearly have put effort into the analysis. But the rebuttals read as at least as high quality analysis, and "it's aliens" requires a lot of evidence for me to take it seriously.