... because the Soviets never doubted it.
The race to the moon was a pure propaganda exercise from both sides. The Soviets were getting there but had some setbacks and the Americans got there first.
If the Soviets had had any doubt, if they thought they could smear it, you can bet your bottom dollar that they would have exploited it, shown it was a fake and then sent a rocket up to where it had supposedly landed and shown that it hadn't happened.
They didn't. Because it happened.