Hi Charles! Julianne from Mixpanel Support here.
Really sorry for the frustration and mixed signals; that's not the experience we want for anyone. To confirm, yes, on the latest Growth plan you can buy additional Session Replays above the included 20K, up to 500K per month. That's not Startups-only, and the changelog you saw is correct. Note, it isn't possible to access Session Replays on a legacy Growth plan, but if you update to the current one, you will be able to select the volume.
I also flagged the conflicting info internally so we can figure out what went wrong there and make sure it doesn't happen again. It sounds like some of the internal documentation needs to be updated, so I appreciate you flagging this.