I think it's likely that calibration is domain-specific, so I'm not sure I buy this unless the calibration has occurred in the same domain, which is rare/impossible for the domains we're talking about.

I think you can argue that the probability is inherently unknowable but I don't see how a detailed process is much better than an intuitive process.

It's very useful to have a mental ability to distinguish between 0.01, 0.001 and 0.0001 when it comes to thinking about XRisk events. I don't think that it's a good practice to call all of those events unlikely and avoiding to make semantic distinctions between them.