SOLVED: Never mind, I finally found it. There are actually two issues:
1) There is yet another checkbox labeled "Automatically select the last logged in user" that needs to be unselected.
2) There is a problem with the default Mint-X Login Theme, at least on my computer. When displayed, it only gives the option to login as a non-root user, then when it is too late, it very briefly displays a pop-up where you can select a different user (but you can't select it because the login process has already started). Changing to the "Clouds" login theme fixed the problem.