“modify front end text editor” is not an option anywhere in WordPress. You must have a theme or plugin that’s adding this option. WordPress has no UI for modifying the editor at all.
The problem is that WP Job Manager is probably not using the same ‘front end text editor’ that this theme/plugin is referring to. The theme/plugin that’s adding this option probably uses a filter that applies to certain instances of the editor, but WP Job Manager likely doesn’t use it that way, which means that whatever filter is being used isn’t applying to it.
If you want to modify the WP Job Manager front-end text editor you’ll need to ask its developer, or check the documentation, for a method to do this.