They say "prerelease" Safari because it's the version they're marketing, but they test it before they officially release it so the marketing materials are ready. That version of Safari would've been current at publication time.
That's a good point: if they are literally about to release it, it's much more similar to stable than beta.
I tried to look at historical timing, and it's kind of confusing. For example, Apple said they tested "prerelease Safari 14.0.1" in October 2020. But the initial Safari 14 came out on 2020-09-16, an 14.0.1 was a security update released on 2020-11-12 (which I would expect not to have any web compatibility or performance changes).
My overall takeaway is that what Apple is doing here is unremarkable and not misleading.
