Intensity and ascent are not strictly related. My rides with what I consider to be "climbing numbers," that is, +60ft/mi ascent, routinely have lower kJ/mi numbers than flatter rides or rides on undulating terrain.

63.1 miles with 3,800ft of vertical, 36.5kj/mi (17.3mph)
35.4 miles with 770ft of vertical, 35.4kj/mi (20.8mph)
43.6 miles with 1,184ft of vertical, 39.1kj/mi (18.9mph)

Going solo +22mph on flat land is as hare as tackling +6% grades, if not harder. Because there's no downhill on the other side.