It doesn't matter that it's on the holdout, he's partitioning an already small dataset into 5 partitions and talking about the accuracy in using 80 points to predict 20 points. The whole argument is usually that in the law of large numbers you can now have a statistically significant difference in accuracy. When you're predicting 20 points each with 5 (potentially different) models you likely don't have enough to talk about statistical significance.