The allegations here, if I got it right:
1. One guy who worked for Saudi intelligence knew about the attacks (disputed).
2. 15 of 19 attackers were Saudi nationals (dubious relevance, generally we don’t blame a whole country for crimes committed by individuals from that country).
3. Bin Laden family has close ties to the Saudi royal family, and Al Queda got funded by the Bin Laden family.
My surface-level takeaway is that Saudi royals deserve criticism for associating with the supporters of terrorism. However, I don’t see evidence the Saudi state conspired to commit the 9/11 attacks.