What Trotsky took to be the hard underlying reality, Shachtman considered a pretence or facade. There was no market in the Soviet Union. Shachtman never argued this issue out, as far as I'm aware, or even mentioned the difference between his view and Trotsky's. But I guess Shachtman's thinking here is connected with the fact that as late as 1961 he was repeating Trotsky's assessment of capitalism as being in hopeless decline. But I believe it is false.
Capitalism is never self-regulating.
Alongside that regulation, markets are economic regulators, within limits. Conversely, the Stalinist economies were often extremely resistant to political will. I n the early '30s Stalin's regime seriously talked about replacing wages by state rations. W orkers in the USSR and in most other Eastern Bloc countries moved from job to job, seeking the best bargain. Employers bid for the best workers with bonuses and perks, and tried to squeeze productivity out of their workers with piece-rates.