Apart from the sheer impracticality of the statement, anyone who suggests that release management can only be done if there is a single repository is just not trying hard enough. When it comes to software repository management, it's easy to say there will only be one repository but that's not so easy to stick to.
Migrating the codebase from the developer's repository of choice is error-prone and usually results in unwanted compromises over things like how many revisions can be kept. Retraining the team on a new software repository is expensive and retooling the team can be prohibitively costly.
What software development teams do need is the ability to coordinate their activities irrespective of the repository each member uses. They need to be able to move the code from the development repository to the test areas repeatedly and automatically. They also need to safely deploy code to production. It's not necessary to disrupt the development organization and spend money on software repository management solutions developers will resent using.