"Iraq exists only as an idea, not a nation." What do you think of the author's assertion?

Yup ... dead on.
Iraq never really existed until 11 November 1920.

The British established the "State of Iraq" under the Hashemite king, Faisal I of Iraq, who had been forced out of Syria by the French.

British authorities selected Sunni Arab elites from the region for appointments to government and ministry offices.

Iraq is approximately 60-70 percent Arab Shi'a, 20-30 percent Arab Sunni and 10 percent Kurdish.

Iraqi Kurds are mostly Sunni, with about 10% being Shi'a Faili Kurds.

So from the get-go the west had enforced a minority rule of the area.
What could go wrong?

There are three nations inside the 'border' of Iraq. This was done intentionally by the Brits in Iraq and Afghanistan when carving up the old Ottoman Empire, but other also by Imperial Powers elsewhere. When occupying a foreign land its best to keep factions fighting one another so as to not fight the Empire. Find a roughly 1/3 minority, raise them to power to control the government, military and structures of power, make them dependent on you and they become fierce fighters to perpetuate the status quo against the other 2/3s. This all works until there is no Imperial Power to support that 1/3, or until you decide the strongman that inherited that 1/3 after the Empire needs his regime changed. Then the inherent imbalance in the 'country' you drew will eventually re-stabilize through war: it will either split along ideological lines into several sub-nations, or one group will rise to dominate the others (often brutally). ISIS/ISIL/Daesh knows their history, we seem to have overlooked it.
https://www.google.com/search?q=Sykes%E2%80%93Picot+ISIS+ISIL&ie=utf-8&oe=utf-8

This is some of the truest things I have ever heard about Iraq. I think it is dead on.