I’m not sure about the details here (I haven’t read the paper, yet), but this kind of thing isn’t unusual in taxonomy, I speculate it went something like this:

Someone collected a sample speciement 25 years ago and it was either not examined or misidentified as another Atrax spp..

Then years later someone re-examins the sample (or maybe they found a new speciement in the wild and then compared it to existing samples) and realised and moved on to formally describe it.

Full paper describing the new species is availible here.