You're handing over a delegated private key, not the actual private key. It's kind of like a proxy version that doesn't mean the real one can be accessed by anyone. If it could you can bet we'd hear about it.
This is out of date regarding local harvesting but it gives a description of delegated harvesting which is what people do through the nano wallet - https://blog.nem.io/how-local-and-delegated-harvesting-works/
Thank you, now I understand it and deleted my tweet.
So, if I use Nano wallet and my delegated harvesting is set up and ACTIVE I shouldn't give this information?