I can't find anything that answers those two questions, but a lot of articles say it's only the second species of monkey they've found in 28 years. Since we didn't know about those for so long, there could be several more we don't know about.
"We" here obviously means the western world, as if I remember correctly, the natives of that area very well knew the monkeys.
But those are relatively small monkeys, big apes is another thing. When's the last time we discovered an ape? Could they even live where they are supposed to live, wouldn't we already have found them since they need a certain population to sustain?
I'm just saying it is very unlikely from a biological point of view they even can exist, even more so without us knowing about them. And there have been so many hoaxes of this, so many stupid people, that they would need really solid evidence for me to consider it anew.