Hello. According to the documentation on Self enrolment
- Enable existing enrolments - if disabled, all users enrolled via this plugin can not access the course.
- Allow new enrolments - disable if you do not want to allow students to self enrol in the future.
If you set the Enable existing enrolments to NO, then for that particuar enrolment method in that particular course, nobody can get in. It would be the same as disabling that method in course administration, yes (but not Site administration) as you are only disabling one enrolment method. You could have other self enrolment methods in the course as well as that one, potentially.