I'm a bit confused as well.
What I gather from the original statement is that the aircraft climb gradient is relative to the horizon , so on a uphill slopping runway you will have a lower (relative) obstacle height, and you need a lower climb gradient; if the the runway slopes downhill, for the same obstacle (absolute) height and position, your aircraft constant climb gradient will be affected because your relative height to the obstacle is indeed higher and so is the the climb gradient needed to overcome that obstacle. This can be useful when calculating the MTOW for that particular rwy, is it not?
GD&L