Travel wasnt as common or as free as it is today. Russian and Western societies were completely cut off. The only real sources you would have is word of mouth. The word of mouth was likely to be a reporter or an embassy worker. Otherwise the odd tourist, but I'm not sure how a tourist was supposed to gauge the impact of Grapes of Wrath in another country and/or why a tourist's word should be held above those of reporters/embassy workers.
The reality is that everything we know about life behind the Iron Curtain since it came down indicates that there is no reason this couldn't happen. Cars were extremely rare and expensive in the USSR. Cars were significantly cheaper and plentiful in the United States. The only point of contention is whether people living in the USSR watching a movie about the United States would be fascinated by the fact that even the poor in the United States had cars, something even middle class to rich people in the USSR could not obtain easily.
That seems a fairly narrow point anyways, whether true or not.
A named embassy source recounting how they heard the story, or ideally some journal released post Cold War from some associate of Stalin.
I wasn't overly skeptical that the film was censored, possibly for the reasons mentioned, but the article makes it sound like it was some huge USSR move that Stalin was behind.
That's what makes it look suspicious when stated as fact. I could imagine it being in someone's memoir, though, but the anecdote wasn't attributed to a person.
Some of his/his associates documents have been published post fall of the USSR, so non-zero but low. That's why the claim seemed extraordinary.
My wife was born in the USSR just before its collapse and grew up in a post-Soviet country. Her family was pretty poor, and they didn't get their first car until the late 90s. My wife was a young girl then, so only has vague memories of needing to hitchhike to grandma's, but that means her parents went almost 40 years without a car. And their story is not unique. Pretty crazy.