If you swap in the extension cable head, that does indeed have a ground pin, at least in Australia anyway. The grounding comes from that metal ring that the connector uses as a guide. https://www.apple.com/au/shop/product/mw2n3x/a/power-adapter...

only two prongs of which make it through. Usually the regulation as I understand is that it's fine if you can prove the case can never get in contact with anything electric, for most laptops that's just being made of plastic.

As has been established in other threads here, the metal button thing the prongs slide onto is an earth connection.

> only two prongs of which make it through

The big recess above the pins is what encases the button of the charger and provides grounding if it includes metal strips. Assuming the charger itself has a metal button.

In the EU a grounded cable has been the default forever (I have a grounded cable from my 2010 MBP which I use as travel cable for my 2021 MBP)