Let’s start with your 694 nucleotides sequence within the last common ancestor of humans and gibbons. And let’s say that 4% of the positions within this sequence have changed by drift during the journey leading to humans. Now, what is the probability P that a given mutation that has occurred along the branch leading to human also occurred during the journey leading to gibbons? As a rough estimate, can’t we say that P is more or less equal to (0,04^2), ie,. 0,16%?
According to the above reasoning, we would expect no more than 1 or 2 cases of homoplasy in your example. But we observe 22 of them.