The only way to stop "extremism" is for Sunni countries to actually help muslims and defend Islam. If they fulfilled this simple obligation the allure of non-state actors would diminish and indeed they could be brought back into the fold. We need rehabilitation, reconciliation, and centralization. If thats what he means then good.
But if he means tolerating the ruining of religion and basic human decency like "moderate" means in other countries - or bowing to Iran then it's big problem.
He wants to discredit ISIS and Iran and lead Saudi investment in Iraq. He was told by the US that the only way to prevent a full Iranian takeover of the country is to invest in it. And btw Sunni radicalism as known today is a direct reaction to the Khomeini revolution, MBS knows that. And when he says extremism he means Shia too - this was made very clear during the Trump conference in Saudi during Summer.