At the time, in Greece they thought of apes as monkeys without tails. They also had no reason to think those creatures were particularly bipedal. Or that there was any particular relation to humans. Aristotle was describing Baboons, which walk on all 4s. To Plato, a bird might be the only other creature that walked on two legs. It also has pink skin for what that's worth.

It's easy to forget that the foundation of knowledge we have is so incredibly vast it would be incomprehensible to the ancient Greeks. We learn in elementary school things that people wouldn't work out for centuries.

Imagine telling Diogenes that dolphins are foxes that learned to swim. Or that the giant skulls they keep finding aren't one eyed giants, but the skulls of ancient hairy elephants.

Plato was alive when Greek philosophers decided the earth was round, and it would be a few hundred years before somebody would make the first real calculation of its size.

I guess they just forgot about gorillas

We often take for granted how much information we have about the animal kingdom. Gorillas are native to central Africa; it is highly unlikely Plato ever even met somebody who had seen one. What makes Greek philosophy impressive is how much they were able to accurately describe given the incredibly limited slice of what is now “common knowledge” available to them, and how durable the methods they employed have proven when provided better priors.