I don't think this is accurate. Sure, no human can understand 500 billion individual neurons and what they are doing. But you can certainly look at some and say "these are giving a huge weight to this word especially in this context and that's weighting it towards this output".
You can also look at how things make it through the network, the impact of hyperparameters, how the architecture affects things, etc. They aren't truly black boxes except by virtue of scale. You could use automated processes to find out things about the networks as well.