This confused me, because there's definitely no pin for an earth ground in an USB connector. So I decided to repeat your experiment and see which pin it uses.
Turns out, it's the cable's shield that carries the earth connection.
That's really cool, so the connection ground is in the cable, but the earth ground is the connector itself? Is that part of the spec or just something which sometimes happens?
I don't know if it's specced. I couldn't really find anything more than the pin diagrams on Wikipedia ( https://en.m.wikipedia.org/wiki/USB-C ). Those don't mention earth ground at all.
But since both Dell and Apple are doing it, I guess it's at least an optional thing they can do.
Turns out, it's the cable's shield that carries the earth connection.
Always cool to learn something.