Bioko is in the center of the chimpanzee range, where no mammal fossils have ever been reported. It is outside the reach of mainland African viruses, such as CERV1 (PtERV1). It is always warm on Bioko, both in the water and on the land, and it is nearly always cloudy, so body fur and top-head hair (think of male pattern baldness) is not needed for sun protection. There are no large predators on Bioko, even today, so apes could learn bipedal running without danger. It is a huge island, where humans could evolve with gene flow and without mixing with other apes for a few million years. Take an aerial tour around cloudy Bioko here: https://m.youtube.com/watch?v=wCf3rOIk14I
We don't know what Proto-Bioko was like when chimpanzees may have stranded there a few million years ago. It may have been like Bioko today, or it may have been a tiny barren volcanic island with no trees and almost no animals (like Galapagos was when a few animals arrived there by rafting.) But even if it was exactly like Bioko today, and the chimps stranded on a small rocky islet like this: https://m.youtube.com/watch?v=-0wtDWx6mPQ, they could not swim to the main island, and would find no food on their little islet except marine food. Their descendants would eventually learn to swim, and when they swam over to the main island, they would maintain their attitude that the only real food is marine food. That is similar to paleoanthropologists who maintain their attitude that the only real evidence is fossil evidence. That is how chimps and people are.