The second sentence in your quote gives you the TL;DR:
The theorists are unable to decide [...] what States actually are.
This is more than just a conundrum for writers of philosophy. If you want to have a conversation with anyone about what "the state" should be responsible for, what its boundaries should be, what limits there should be on its powers or whether it should in fact exist at all, you need to start by agreeing what "the state" actually is. If you can't do that then you are going to be talking past each other.
To illustrate the problem, lets look at a practical example. Is a commercial company part of the state?
Mostly the answer would seem to be "no". Anyone can create and operate a company, and the only interaction with the state is formal registration and subsequent payment of taxes. The state doesn't tell a company owner how to run the company beyond a set of laws that must be kept, and if the company runs out of money the state will just let it die.
However for large companies the distinction between state and company often seems to break down. Large companies frequently exhibit a selection of the following characteristics:
They have a majority of the market, or in some cases even a government-guaranteed monopoly.
They are subject to rules and regulations which do not apply to other companies. Often these rules require them to do things (such as serve some customers at a loss) which further government interests rather than their own commercial interests.
They are provided with subsidies by the government, to the point where this is a major source of revenue.
Employees frequently move between company and government.
These characteristics mean that a big company can behave like a division of the government; it is funded by taxation and carries out government policy. Indeed, if the company were officially made part of the state by nationalisation the only people who would notice would be the senior management and shareholders.
At what point do we consider such a company to be part of "the state"? The answer matters because if we define a company as outside the state then the limits on its behaviour that we place on the state do not apply to it.
For instance, look at the current argument about free speech on social media in the USA. The social media companies are not part of the state, and hence are not required to observe First Amendment limitations on state control of speech. Indeed, their separateness from the state means that they have their own First Amendment rights to decide which speech to carry.
But on the other hand look at the case of Marsh vs Alabama in which Grace Marsh, a Jehovah's Witness, won the right to distribute religious literature on the privately owned main street of a company town. The Supreme Court held that
In our view the circumstance that the property rights to the premises where the deprivation of liberty, here involved, took place, were held by others than the public, is not sufficient to justify the State's permitting a corporation to govern a community of citizens so as to restrict their fundamental liberties.
In other words, because the company was acting like a state in providing a public street, it would be held to the same constitutional limits on its power to regulate speech within that area.