Superpower status is not solely determined by economic strength. Actually, it's not clear how to define a superpower at all. Most of the time there's simply a tacit understanding among policymakers, international relations theorists, and political scientists that some states are more integral to the stability of the world order than others.

'Superpower,' as we understand it now, picked up in usage right after World War II as the incredible military mobilization of both the USA and the USSR made it clear they were in a league of their own when compared to the older European counterparts, whose empire was crumbling. This int'l situation would be the subject of study by such people as Hans Morgenthau (not important to know who that is). But the superpower label could just as easily be applied, and was, to the British Empire before its decline. The British Empire, the USA, and the USSR had very little in common to give a good definition for what a superpower is, except for one thing: military might.

That's basically the answer to your question. The Soviet Union had an unrivaled conventional military force on Eurasia and a position astride every geopolitical hotspot on the planet (just glancing at a map, the Eurasian heartland let the USSR threaten pretty much any old world country.) This was the result of both World War IIs legacy and the heavy central planning that was key to the communist system. The Soviet Union had a bad consumer economy, without a doubt. There weren't efficient market forces for consumer goods, and up until its collapse the country was never self-sufficient on grain. But massive, industrial projects benefitted from a centrally planned system since resources and labor were easily allocated toward them. Virtually all of these industrial projects were for military purposes, hence the USSR's military rivalry with the USA that lasted despite the USSR's deeply flawed governance system and lack of markets. Magnitigorsk would be one example of how central planning could produce incredible works amongst a weak overall economy.

The entrance of nuclear weapons into the mix complicated things further. The true acknowledgement of the USSR as a superpower could be considered the trip Richard Nixon took to Moscow in the 1970s: the subsequent period of Detente was something of an acknowledgment that despite its deficiencies compared to the USA, no foreign policy decision could ever be made without considering the actions of the militarily powerful, nuclear-armed USSR. This reality had been around for a while (most obviously driven in by the Suez Crisis) but Nixon seemed to understand treating the USSR as the superpower it wanted to be was probably better than ignoring it. The issue of the USSR economy never really solved itself, although from what I know oil reserves allowed it to pursue the military prioritization and central planning all the way up until the collapse.

So, to achieve superpower status, there simply needs to be an acknowledgement among the int'l community that nothing can be done without checking in with the alleged superpower first. That's mostly because of military posturing and the possibility of causing a conflict or massive war if the superpower's attitude isn't taken into account. The economy, while important, can be substituted by a massive army or modernized nuclear arsenal, and an active foreign policy. Whether a superpower can keep its status without a complementary economy is another matter (this doesn't look to be the case).

Hope this helps. Basically, looking at what we historically consider superpowers, the criteria isn't really economic. The USA could do nothing in certain places like Western Europe or Africa without thinking what the USSR was planning to do, and vice versa. Many nonaligned countries struggled to placate one or both of these superpowers during the Cold War, because of the military support necessary to sustain a young country as it went through decolonization or revolution (which happened in spades after WWII). And nobody wants a war.

Someone may come along and disagree with me, so to clarify why I wrote this the way I did, I find discussing the idea of a 'superpower' more key to this answer than what a more strict historian might. I don't like to go back before the Cold War, for example, just because I don't think it wise to super-impose the superpower label onto more and more historical empires just to find more case studies.

Sources: The Tragedy of the Soviet Union, by Malia Politics Among Nations, by Hans Morgenthau A Superpower Transformed, by Dan Sargent The Superpower Quest for Empire: The Cold War, and Soviet Support for Wars of National Liberation, by Roger E. Kanet.