I agree with you. The country went from a civil war and foreign invasion (including by the US, something I'm sure 99% of Americans don't even know) and then 25 years later were invaded by an alliance of Germany, Finland, Italy, Hungary and other countries. Followed by a Cold War against a nuclear armed opponent and its western European allies.
Yet during the 1930s, when starving veterans were being shot dead in the streets of Washington DC, when unemployment in the US and Europe was over 20%, the Soviet economy was booming. Massive steel factories were erected in places such as Magnitogorsk, and factories building tractors to send to industrialize agriculture. The US said Russia stole all technology from the US - until Russia launched Sputnik. You can't blame the adversary for stealing IP when they're doing things you can't do.
China also was a rural backwater, dominated by Europeans and Japanese until the 20th century. Under the leadership of the politburo of the communist party, its economy has been growing 10% a year since the early 1980s. People say that is unsustainable, but they've been saying that since the late 1980s. I'm sure their growth will slow down eventually, but they're already the second largest economy in the world by GDP. Of course, the politburo is not as left wing as it was during the height of the Cultural Revolution.
The USSR economy did stagnate when Stalin died, and people like Molotov were sidelined. Capital spending decreased, policies like detente were enacted, and the economy stagnated. The economy did very well before this change in policy though. American and European workers and firms went to the USSR in the 1920s because there was work and contracts in their growing economy, while the US economy went off the hinges. Kind of like the current US unemployment rate which is higher than anything since the mid-1980s.