"GPT–6 Astra mentions a private collection, but it is not clear what this is"!, my spidey senses makes me think it hacked something? Or am I misreading this?
A private archive doesn't necessarily mean a non-public archive. Simplest reading is someone rehosted the files.
As somebody who has spent a bit of time accessing archives both in real-life and online for history research, my natural reading of 'private collection' is privately-owned not private as in not-accessible to the public.