They are, but using the same process for all books is simpler and cheaper.

Yes, that is what the person you are responding to is saying. That's why they are questioning the assumption that this practice extends to 18th-century books that are solidly in the public domain.

If it is happening, it is an outrage. However, the 404 article doesn't actually provide any evidence of this; it just connects the shredding of digitized books and the digitizing of rare books to the assumed shredding of rare books, which isn't necessarily happening.

Though they're not shredded simply due to copyright, they're also shredded due to cost, speed and the quality of the scans.

Just sociopaths doing sociopath things. Bugger civilization.