The NMS algorithm given here is non-traditional at best

[jdhao]

There are two main differences with the well-recognized NMS algorithm.

First, there is no scores for each bounding in the algorithm here, where in the well-recognized versions, scores for each bounding box must be provided.

Second, the overlap calculation here, does not calculate the IoU. The union of area of two boxes should be area1 + area2 - intersection

The authoritative implementation by Ross Girshick is here.