It depends on what person actually means by state capitalism.
If he/she means that state capitalism is when state appropriates part of the surplus value that he/she produced, than in his/her eyes SU even before Khrushchev sold out MTS was state capitalist.
And you can't convince him/her otherwise.
It all comes down to should worker receive full value of his/her labour, or not.
If person believes that should, than he/she advocates for direct ownership of the means of production, which means that SU for him/her is even more frightening than old fashioned capitalism, because he can't possibly run from this new total capitalist, this leviathan.
If person accepts that part of the surplus value should be concentrated and invested in something (science projects, free education, free healthcare, military or whatever), than he/she can be convinced that SU was not capitalist, or at least not capitalist in a traditional sense.
As I see it, it was one big corporation (or cooperative, if you like) by the name "USSR" with CEO Stalin. This corporation employed people, but said people were also the only shareholders.
This corporation operated in market environment. Players in said market were corporation "USSR", agricultural cooperatives, artels, and self-employed individuals.
All possibly commodifiable means of production were owned by corporation "USSR", other players in the market rented means of production from corporation, or were given MOP for free use. Corporation also enforced the law by which one person can't employ another.
Corporation managed the only unit of exchange which was used in said market, as well.
So there was commodity production, and so there was law of value in operation. But only in the sphere of end consumer market, because it was the only market. There was no market of MOP, no labour market, no market of intermediate/semifinished products.
Corporation "USSR" sold it's goods for fixed prices, which were calculated as cost price + fixed surcharge around 5% to cover operational costs.
Profit came in the form of tax on circulation.
Does law of value ruled over corporation "USSR"? I think not, because profit never was an ultimate goal, nor was it regulator of production. Plan ruled supreme inside the corporation, and ruled only partially outside.
As to why there was end consumer market, there's two points of view.
One, that of the CEO of corporation, Stalin, is that there was market because there were different forms of property in the economy, namely cooperative form of property and property of corporation "USSR".
Another point of view, that of the Ernest Mandel, one of the leaders of the 4th international, is that there was market because there was scarcity of goods.