Doesn't change the fact that whatever probability you previously assigned to finding Planet 9, it should be one fifth of that now. (Sort of. Not so drastic if you have strong reasons to think there is a Planet 9)
The suggestion is maybe part of the sky is easier to survey, so it was completed cheaply, even though that part wasn't the most likely part. I don't know which way it is, but it's an interesting question, and that's what OP is getting at for understanding the relevant probabilities instead of the relevant area.