I can't find that text of the proposed amendment, but based on the description in the NY Times, there doesn't seem to be any explicit mention of Saudi Arabia, just an additional stipulation under which foreign governments can be sued in American courts, the stipulation being something along the lines of "if a foreign government was involved with an attack on American soil killing American citizens, their immunity does not apply."

This is the existing law:
https://www.congress.gov/bill/94th-congress/senate-bill/3553

There are already several instances where the immunity already does not apply. For example:

> Stipulates that a foreign state is not immune from the jurisdiction of U.S. courts if: (1) the state has waived its immunity either explicitly or by implication; (2) the action is based upon specified commercially related activities; (3) the action is based upon rights in specified property, connected with commercial activity, taken in violation of international law; (4) the issue is rights in U.S. property acquired by succession or gift or rights in immovable U.S. property; or (5) money damages are sought against a foreign state for personal injury or death, or damage to or loss of property, occurring in the U.S. and caused by the tortious act or omission of the foreign state or its official or employee acting within the scope of his office or employment.

So, I think this is a pretty small and reasonable change, to which Saudi Arabia is reacting so extremely because, as "everyone knows" and as likely revealed in the unreleased portions of the 9/11 report, the Saudi royal family and/or people in the Saudi government were, in fact, involved in the events of 9/11.

The Saudis, of course, have to give a somewhat-reasonable sounding diplomatic explanation of why they would be "forced" to take their assets out of the country - that's how the game is played. Since we know that there are already holes in the immunity that could potentially leave the Saudis open to "unlimited damages" (I don't see any indication the damages are unlimited), we have to conclude the Saudis believe they will, in fact, be found culpable in this instance. Now, you can choose to believe that the Saudis believe that because they believe the US court system is just grossly unfair, or because some of the royal family or the government actually were involved, and they're afraid they would be correctly found culpable. I find the latter more likely, given the information we already have, personally.

It goes without saying the Foreign Minister is going to provide a reasonable-seeming cover: that's his job. Selling off 750 billion and destabilizing your own economy is not a rational action. Every billionaire is at the risk of his assets being frozen and seized if he committed a crime. So is every corporation. So, really, are foreign governments, regardless of whether they were complicit in an act of terrorism, given the immunity is not absolute. Nobody else is threatening to take their ball and go home. So why, then, would SA be threatening to do this? Because it makes for a substantial political threat to the US to pressure the US government to not hold them accountable.

In the end, I suspect it's really about a loss of face for the royal family, because realistically, even if sued and found complicit, it would be incredible if they were forced to give up a few billion, let alone hundreds of billions.