We at least have in common with them that we are mammals. Therefore, we can very much assume that our brain is more similar to theirs than, say, an octopus' brain. Apart from that, we very much know how certain parts of the human brain work, and there is no sign that backpropagation is going on in there. And I'd rather argue that parts of our brains are similar to RNNs than to transformers. Transformers rule over RNNs because we are better at training them than RNNs, but brains learn completely differently.