Any source?

Edit: Answering myself: https://gwern.net/doc/algernon/2009-silberberg.pdf

However,

* we see no videos of the two men doing the test. Notice how the ape barely looks at the task, while the men maybe were highly concentrated.

* The men had the goal to improve and beat the ape, the ape probably did not have that goal.

* The men created the circumstances themself, so it may be possible the knowingly or unknowingly gave themself better conditions than the ape.

* The men did up to 2500 units of training, it is unknown how much the ape got.

* We can see that the ape does nine numbers in some tests, the men did just five.

* the men propably relied on language to learn the task, the ape did not.

So, this is quite a flawed study too.