In that light I'd argue that the USSR was state capitalist, in so far as instead of having a multiplicity of competing capitalists they had the state acting as a single giant capitalist.

Capitalism is not defined as simply "not communism". The very idea of "state capitalist" is essentially an oxymoron. Capitalism is defined as the control of the means of production is privately owned, or largely privately owned.

The state is not a private individual, it is a centralized monopoly on violence with control over a sovereign area.

Capitalism requires the existence of the capital market be it official, or unofficial. State capitalism was essentially a term invented by anarchists to attempt to discredit Marxist-Leninists and associate them with terms normally applied to capitalists. Its definition displays quite a lack of rigour, which probably explains its perennial use by Noam Chomsky and insane American Libertarians. A quote from Lenin is particularly revealing of why Anarchists wanted to invent the term, as it eventually became associated with other forms of Leftism, even to their adherents:

Reality tells us that state capitalism would be a step forward. If in a small space of time we could achieve state capitalism, that would be a victory.

However, this theory only holds up if one views the State as a series of colluding private individuals rather than as a separate entity within itself. As I think is quite clear, the State is an inherently separate entity from the individuals that constitute it, which is why the sovereign state is referred to as the "Leviathan". The State itself utilizes capital not only to acquire more capital or to consumer goods, but also to enforce organization, potentially expand, and prevent incursions upon its own monopoly on violence. All these things make it a fundamentally different entity than private individuals, and means that it can't itself engage in the practice of capitalism. Capitalism can't be singularly defined as the "extraction of surplus value", and just because a system has some capitalist characteristics doesn't make it capitalist.

The term "State Capitalism" is just another example of when anarchists attempt to disassociate with and denigrate other Leftists by applying and inventing terms associated with capitalists to describe them. It would be much more appropriate to define such a society as either state socialist or simply statist.