I've read through the order again. I can't find anywhere where Alsup says the destruction was required.

He cites three cases where a conversion from one format to another (without destruction of the previous version) was ruled to be fair use. Including scanning books with the google books case. (And referenced the Napster case, where a similar argument was rejected)

Then made the following comparison.

"Here, every purchased print copy was copied in order to save storage space and to enable searchability as a digital copy. The print original was destroyed. One replaced the other. And, there is no evidence that the new, digital copy was shown, shared, or sold outside the company. This use was even more clearly transformative than those in Texaco, Google, and Sony Betamax (where the number of copies went up by at least one), and, of course, more transformative than those uses rejected in Napster (where the number went up by “millions” of copies shared for free with others)."

So it wasn't transformative because of the destruction. The destruction only made it "even more clearly transformative" than those other cases.

Like, how can destruction be required if there were previous cases where it wasn't?

The key legal point is not that Anthropic destroyed the books, but the key fact was that Anthropic didn't distribute the scanned copies. Alsup keeps returning to this point:

"But what matters most is whether the format change exploits anything the Copyright Act reserves to the copyright owner. Anthropic already had purchased permanent library copies (print ones). It did not create new copies to share or sell outside"

"But again, the replacement copy here was kept in the central library, not distributed"

The conclusion of that section doesn't even mention the destruction at all.

arstechnica isn't exactly wrong, the quote also mentioned "and kept the digital files internally rather than distributing them". It just put way too much emphasis on the destruction, and not enough on the lack of distribution.

The other thing that arstechnica are missing:

Antropic didn't destroy the books because they thought it would strengthen their legal argument. They destroyed the because it's a lot cheaper and faster to scan books by ripping off their bindings and feeding the stacks of loose pages into a document scanner.