@BestCodes
Thanks the suggestion to use the Tab key for page context. When I did, Aria gave me the page summary option I was looking for.

The weird thing is that Aria was previously suggesting other menus and functions that didn't exist in my computer's Opera browser, but did exist in my mobile version of Opera (both browsers were updated versions of Opera). My conclusion is that Aria wasn't trained on the most recent desktop computer version of Opera, but only on the mobile version.

Even stranger, After updating Opera on my Mac, Opera's new features page specifically prompted me to explore the Aria page summary feature. Though I followed the specific instructions outlined by Opera, it only resulted in unusual error messages. I guess the new page summary feature wasn't tested on all platforms.