If done correctly, the entire description section (in the Edit Course settings page) should disappear from their view. I wonder if something else is conflicting. So....
1. pick a teacher who ideally should not be able to make that edit to the summary
2. go to a course they are a teacher in
3. In the course admin Block, click on 'users', the. 'permissions', and then 'check permissions'
4. Type that teacher's name in the search box, click on their name, then click the 'show this users permissions' button
5. You will now see all ththe contexts this user has in your Moodle.at the top, see if they do have another role that does give them the permission to edit the course summary. They should be authenticated user in the system and teacher in the course. See if there is anything else. Also on that page, do a search for (course:changesummary) to ensure it has No next to it
Maybe there's a better or faster way to figure this out but this is a start.