“The crucial innovation that made for development of the state was the idea of the corporation as a legal person”
The idea of a legal person is clearly older than capitalism; the status was granted to guilds and public associations long before capitalism. The idea of corporation is however more closely related to capitalism and the idea of limited liability and legal personhood as applied to corporations is quite unique to capitalism or at least has become a central concept only in developed forms of capitalism.
Thus to say that the idea of a state as a legal person is something new is basically to say that a capitalist state is something new but to draw a conclusion from this that the state as such is new is to conflate a capitalist state with the state as such. The capitalist state as a new form of government is something new in the sense that compared to previous forms of the state the capitalist state is unparalleled in terms of increase in the concentration of power and its reach but that does not mean that the concept of state is something new. This might be going on when some theorists claim that the state didn’t exist before the modern period.
I think the basic distinction we should make is the distinction between the government and the state. A Government is a relatively temporary form of governance while the state is a permanent form of governance. We can make further distinctions within the concept of the state itself. We can differentiate between the state as an institution and the state as a form of rationality of government or a form of legitimacy, a general form of the legitimate use of power or coercion within society. All these distinctions can be found before the emergence of the modern capitalist state as a new form of statehood.