You are correct - they are protected methods. But when you run Queue::getSqs() - it is returning the underlying SQS instance - which means you have access to those methods.
So this works:
$list = Queue::getSqs()->listQueues();
As does this:
$attr = Queue::getSqs()->getQueueAttributes(array('QueueUrl' => 'string'));
A full list of available commands is here at the AWS website.