"Storage performance is up to twice as fast, with a next-generation SSD architecture, delivering industry-leading read and write speeds for rapid project loading, file transfers, and loading huge LLMs."
Here's the original, archived press release from August 25th (look under "Blazing-Fast Storage and Pro Connectivity"):
https://web.archive.org/web/20260825130329/https://www.apple...
And here's the first modified version that the Wayback Machine has, from August 26th:
https://web.archive.org/web/20260826020313/https://www.apple...
So it seems like Apple made a mistake in this case. I agree with you that it will likely be PCIe Gen 5 - that's much more logical.