The “default email mailing list mode” setting that is enabled in your screenshot accomplishes this. When you enabled it, a popup window should have opened that has a button with a description asking if you want to enable the setting for all users “historically”. Selecting that option will update the email preferences for all users on the site, with the exception of any users who have previously disabled mailing list mode from their preferences page. Since that option wasn’t displayed to them previously, this means that all users will now have mailing list mode enabled.
If you didn’t click the button that applies the changes historically, you can disable the “default email mailing list mode setting” from the site settings page. After saving that change, re-enable the setting. The button to apply the changes historically will be shown to you again.