Getting order extension attributes from Swissup Firecheckout?

I tried getting the checkout fields attributes which are included with the Swissup Firecheckout … the developer said its possible with $order->getExtensionAttributes();.

So I created some throw away code to test, like this:

$orderId = 12345;
$objectManager = MagentoFrameworkAppObjectManager::getInstance();
$order = $objectManager->create('MagentoSalesModelOrderRepository')->get($orderId);
var_dump($order->getExtensionAttributes()); // does NOT work
var_dump($order->getIncrementId()); // does work

Nothing is dumped except for the increment id of the order. I am sure the order has some checkout fields set …

Am I missing something or whats wrong here?

Thanks!