I've been doing this now since 2003 and have found that height doesn't matter all that much. They even find the way to transfer the eggs in a 40g breeder which is low. This has been consistent with five species of seahorses.
A seahorse will not die due to a low height in the tank.
When I first started, the higher tanks were the recommendation and indeed many still do recommend so, but it is NOT a requirement for survival or mating.
In my various seahorse tanks, I've found that many seahorses DON'T swim all around the tank, especially males in the erectus species that I have in my 90g tank.
I don't frequent seahorse.com much as I prefer the seahorse.org and seahorsesource.com better, especially for more current up to date information.
There have been a lot of changes in the hobby since I first started, and while I keep my seahorses in a manner that works for me, it is only one of many ways to succeed.
However, the great majority of seahorses purchased for aquaria are doomed to die early deaths as their needs are not met by one method or another, many times due to lack of research but also due to improper advice given by unknowledgeable people, especially many LFS's.
All this being said though, it is STILL not a good idea to put seahorses in that sump unless the needs of the seahorses can be met. (IMO)