Setting the required attribute to true for checkbox types means that the checkbox is required to be checked, a.k.a. setting it's value to true. When it is not checked, a.k.a. it's value set to false the content cannot be published.
Steps to reproduce:
1. Go to Site Administration > Content of the default Site
2. Select to Dynamic Data Lists
3. Click on Manage Data Definitions
4. Click on "Add"
5. Add a Boolean field and click on it
Actual Result: In the field settings, you can't see the "Required" option
Expected Result: You should be able to set the field as required