While of course it's historically inaccurate to blame the Grand Mufti for the Final Solution, Netanyahu's statements do hint at a legitimate debate among Holocaust historians - that between intentionalists and functionalists. The extent to which Hitler and other parts of the Nazi leadership initially planned on an extermination program is a point of some contention. There is no question that Hitler and his cronies fostered an intense antisemitism, but extermination really only got underway in earnest a few years into the war. There were plenty of ad hoc massacres of Jews - not least being the actions of the Einsatzgruppen, especially during Operation Barbarossa. But an intentional plan to exterminate the Jews may not have materialized until later, and have been driven by individual initiative from below as much as orders from above.

There is evidence for both sides of the argument, but I think the truth is probably a combination of the two. A fully realized plan to exterminate the Jews likely was not even in Hitler's mind or that of his leadership in, say, 1939. But if you took his argumentation to its logical conclusion (not really requiring all that much in the way of logical leaps), it was an obvious outcome. And functionalism would only have worked with some element of encouragement from above.

So Hitler indeed may have fully developed his extermination plan in stages, and it's certainly vaguely possible that the Grand Mufti advocated in that direction. But to suggest that the Final Solution is an invention of the Grand Mufti is stretching history to the breaking point.