This integration is active on one of my apps - I wanted to test it out and explore its features and potential value in my case.
So yes, I can confirm that Session replay is available 
That’s actualy why I said if feels a bit scary - you can see exactly where users click, which pages they visit, how they move the mousen and more 
Very useful though, especially to check how effective your screen visibility settings are. For instance, I noticed that some users have access to tabs or components they weren’t supposed to see… because I hadn’t set things up properly 
EDIT: If I remember correctly, you need to go to your PostHog dashboard and enable the feature manually: Session Replay → Settings → toggle Record user sessions to ON.