How do you accomplish this in-game?

quit and reenter as another civ

Ok, I was under the impression it was possible to do this during the game.

I have the same question....

What does it mean to "quit and re-enter"?

How, in other words, can you switch the country you are playing as, in the middle of the game?

You quit to the main menu (or disconnect otherwise) and reconnect to the - running - game. Then you get to choose your civ.

Ok, I was under the impression it was possible to do this during the game.

Python popups cause issues, so all of them were disabled.