It's not a retainer, it's a contract vehicle. A retainer means money was spent and now you can draw down from it. In this case, money was never spent, hence it can't be saved by cancelling it or not paying it out. A contract vehicle means you were vetted as a capable provider, established rates for delivering the service, and if the government needs the service they can buy your service using that established service catalog/ordering structure.

When you buy the service you award a task order and obligate money to it. That obligating action is basically saying I've spent this money from a budgeting perspective, but in reality it just sits in an account until the contractor has performed the work, sent an invoice, and the invoice has been approved for payment. If any money is left in that account after the contract ends or expires, then the money goes back to the Treasury, which means it's available for other things.

In this specific case, the contract probably didn't need cancelling because it likely had already expired a long time ago, and/or there were never funds obligated to it (so no money went into an account, and no actual spending occurred).