The scene IS shown in a montage of deleted scenes.
SHOULD have been in the final edit.
Also, without actually SEEING on screen at least ONE of the horrific acts Luca Brasi committed in the book, it’s hard to understand the mortal fear of him.
OK I’m done :)
They would have been better off making a Godfather prequel than Godfather III. II had some flashbacks, but it really didn’t show the details as to how Vito rose to power.