There are two problems:
1. The iThemes Security plugin’s hide backend feature defaults to “/wplogin” for the login url instead of “/wp-admin”. (It can be different than “/wplogin”, but I’m using the default.) The Temporary Login Without Password plugin doesn’t recognize that, so the login urls provided by it are wrong.
2. When I manually replace “/wp-admin” with “/wplogin” in the urls provided by this plugin and navigate to them, I’m directed to the main login screen with the username pre-populated and an empty password field. There are no errors on the page or in the dev tools console.