The function is used to ensure a smart contract is aware of an nft transfer when the save transfer is used. In this way the transaction is not successful when not used. It your responsibility as a dev to implement how to get it out when it finally comes in. Else its still lost

Yes, that's what I thought. The function onERC721REceived is just a simple and even naive way of making safeTransferFrom believe it is safe, when as a matter of fact, there is no guarantee that it is safe to transfer an NFT to the receiving contract. Correct?

If thinking in a dialogue, it's like Calling Contract "Hey, Receiver, are you ready to receive this NFT?" Receiver goes, "Yes, see, I have this onERC721Received implemented. Trust me and take it easy. Just send it over." Calling Contract "Alright, I trust you since you give me this good signal." But in fact, the receiver may not have a way of transferring it out of the contract at all. Is this what's happening?