It seems a key contention of theirs is the possibility that rare books are being destroyed this way, yet the things they cite don't seem to suggest this (based on their paraphrasing), they just throw the following at the end to make it seem like it's occurring to irreplaceable books:

> You can't replace the last three copies of an 18th-century botanical text once someone shreds them for training data. And the judge said it's legal.

Is there evidence of this? Since otherwise they could very well be describing what is only occurring to in-print or non-rare books. (This is a genuine question since their post doesn't shed any light on it.)

There is zero reason to shred 18th century books. Any such books are out of copyright.