Figured it out.
For anyone else stumbling across this topic, there are two different installers on the downloads page. One for Enterprise and one for Studio+. I just grabbed the first one I saw and never thought to keep scrolling down.

I have previously owned an Enterprise license that I let expire. That was about two years ago.
Now I have another project which requires the use of Octane renderer, and I have purchased a new license of Studio+... but when I try to activate it, I get an error message saying I am missing an All ...

Am I the only one who is not able to download the Mac plugin from the first post? (404 Not Found)
Or have I missed something?