if you have a file "templates/core/loginform.mustache" in your theme then all is fine. If you also want to override the "templates/login.mustache" you need a file "templates/theme_boost/login.mustache"
That's the way templates work in moodle. In general you have to place a template in a folder with the full component name and in it the "mustache" file you want to override.
The loginform is a core component so the path in your theme is "templates/core/loginform.mustache"
The login.mustache comes from boost so the path is "templates/theme_boost/login.mustache"
Moodle look for templates in the following order.
- Current theme
- Parent theme of the current theme
What is found first will be used.
If you want to modify the context data that goes into the login.mustache you can override the file "layout/login.php" in the way I described in the last post.