After reading what it does, it seems that the extensions indeed do get updated automatically, even in v20. Not sure if they are silent too, but they very likely are.
All it does is change the value inside the preferences file from the config folder for the next time opera should look for extensions updates by setting it to a couple of years from now which basically disable updating.
The value to change is located under the extensions section, under autoupdate and looks like this:
I can't seem to remember how time written like this is called though so I'm not sure how to convert it into something readable.
This is an three month old thread in the Windows forum. You must want to post in the Mac forum
I searched the site for help on disabling auto-updates, this is the thread I was sent to. I wasn't even aware there was any specification regarding OS, just a general discussion. I would have specified Mac OS in my search if I was provided the option to do so.