The space.com article mentions "The good news is that the vast majority of the behemoths — the ones capable of causing damage on a global scale if they were to hit Earth — have been discovered", so they think a small minority are still undiscovered.

Of course, that's the problem with not knowing what we don't know. If there are any undiscovered, then we really don't know what percentage we know about. The fields of statistics and probabilities can probably prove me wrong, but I'm used to that.