It's pretty simple really. Trump is targeting Al Qaeda, ISIS and Islamic extremism in general. The Saudis are in conflict with Al Qaeda and ISIS. Along with Qatar, it is also a member of the Gulf Cooperation Council which views Hezbollah, Al Qaeda and  ISIS as terrorist organizations. Ditto Pakistan and Indonesia. The US and the GCC's antagonism towards Iran is well known

I wouldn't be surprised if Trump likely has in mind a counter terrorism group that includes moderate Arab States, Pakistan, Israel, Russia and Turkey.