More non-Arab suppliers is one side. The other is that these petro-states today have much larger populations, with much higher expectations, than they did in the 70s. Saudi oil is still cheap to pump, but instead of 6M people, they now have 33M, who each spend 20x what they did, and aren't on the whole accustomed to working for it.
Two things are hitting oil states right now. One is that developing countries, which pay the best rates for oil, are consuming less of it. The remaining increases in demand are from developing countries, but they are following the same arc as developed countries, and on a relatively accelerated time frame. Their consumption will also decline. One thing TE doesn't address is that another factor is the diversification and increase in global oil supplies from new suppliers, including frackers. That's probably having a bigger impact on the traditional petrostates right now. Hoverver the article is mainly looking at where the puck is going, not where it is now.