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)