This is really important, can somebody please answer to my above post?
The answer is no.
Child private key + parent master public key will let you find the parent master private key.
The code you have pasted is for the old pre-2.0 wallets (it's left in there for backwards compatibility)
So pre-2.0 wallets are more secure by performing that additional computation? Why were 2.0+ wallets weakened then?