Er, I think it is. That is, the aim of the rule.
Mind you, Phil is mainly writing this for horses. He maintains that horses get an advantage when riding down slopes. So mounted only get upslope in their bound, and the rear edge is so mounted can ride off the slope, and still get an advantage if their enemy is on the flat. Sort of momentum thing.
So theoretically you could replicate Gandalf riding down the hill at Helm's Deep, with a +2 for being a Brilliant General
So you can say that the elephants are caught on the crest, and can't get enough momentum down the hill.
Hope this helps,
G^is,
JohnG