I think you've missed the point - which is not that Saudi Arabia will collapse for financial reasons - but rather that the corrupt infrastructure which composes the system of organization (it could hardly be called government) of the House of Saud is getting more and more unstable, and less able to be kept together with mere funds alone - since there is already so much money in the empire, that the costs of the corruption are rising at a rapid rate.
Its like, Saudi Arabia has all the oil money it would need, but it doesn't have an ethical/non-criminal infrastructure strong enough to deal with the weight of the corruption at the very top, and also (the article points out): the very bottom. The society has a systemic corruption problem, and no society ever in the history of humanity has ever lasted beyond the tipping point of this corruption. The article warns that this may happen sooner rather than later.
My issue with the article is that it assumes that the Pentagon doesn't run this model, pretty much monthly, in its simulacra exercises. It seems we've forgotten that the Pentagon has an entire army divisions worth of resources alloted to just this task. This is too naive a point of view; more likely, then, is that this article is actually propaganda to mask the fact that the USA and its military partners, do in fact model for Saudi collapse, and are in fact actually prepared for it. That is an even more terrifying conclusion, given the state of the region today, and the degree to which American foreign policy has impacted the stability of the enemy states; where enemy=anyone the Sauds don't exercise corrupting control over, yet, in the region.