I read the doublespend.py code, and I see your point and also why you tagged it "UNDO" -- it can't be used to spend the same btc twice, but it can be used to "unspend" by returning the whole amount back to the sender. 
That is discouraging. What were they thinking? 

Does that mean this is all lies? https://bitcoincore.org/en/faq/optin_rbf/