Thanks for looking in to this.

This is not disabled. I loaded the page a few minutes ago and it gave me the fullscreen popover. It appears about halfway down the page. I just re-loaded to check and there was no popover. When I load the page in a private browser session I still get the popover. So there appears to be a cookie that remembers the dismissal.

e: I cleared the substack cookies and my main browser session still doesn't get the popover but private sessions do.