This is a nice demonstration, although I feel like it would be more useful to show that it works across domains (Which IMHO is the most important part).
Fortunately, tabs created by opening the link using the middle mouse button don't seem to have a useful window.opener. Guess I'll be more deliberate about that habit.
Edit: if I understand correctly this only allows the opened page to navigate the opening page; it does not allow it to do anything else to the opener. Is that correct?
link.html:
<a id="link" href="opener.html" target="_blank">Click</a>
<script>window.opener.document.getElementById("link").href = "https://google.com";</script>
It's a fairly big deal, but not much is being done about it on the browser side of things. It can only really be used for phishing style attacks.
I assumed we were still talking about window.opener.location (which can be modified across domains)