Is this more than a preprint? Usually really long research like this takes a long time to verify, like that guy who claimed to solve the abc conjecture. The article talks about it like everyone agrees this is valid, but is it?
> […] He did stress, however, that the new paper has not yet undergone peer review.
Or I'd be wondering if someone would ever spot the deliberate <https://en.wikipedia.org/wiki/Beatosu_and_Goblu> trick of changing a single \mathrm{B} into a \mathfrak{B}.
Or both.
However, the authors are in close communication with many of the other authors in their extensive bibliography whose work this follows (and in turn who likely be provoked into further work). The progress (seen in the paper's review-like parts and the bibliography) has been towards proving that less and less "gentle" vacuum spacetimes aren't irrevocably upset by gravitational waves passing through them. The bibliography cites a number of pre-pandemic preprints that were in "to appear in <journal>" state. In that sort of academic bubble, which is far from unusual, errors are liable to be found fairly quickly. Also I would guess that the authors revisited their self-cited previous work, which sometimes produces forehead-smacking.
Anyway, in spite of the common image in popular culture, most math results are more roust and can survive a few sign changes, perhaps with some small fixes.