If two h-rects intersect, there must be at least one point that is in that intersection and thus is in the interior of each one.
To be in the interior of an h-rect means that you have a point, p, where the value of each coordinate landed in the range between the UpperLeft and the LowerRight for that coordinate.
So to get a point in the intersection, for each coordinate, the intersection point must have a value that is simultaneously in the range for the first h-rect as well as for the second h-rect.
This means that the two h-rects must have overlapping ranges in every single coordinate.
So if the two h-rects have an overlapping range in every single coordinate then they intersect, otherwise they do not.