They’re not shorter. The front section of the spring is slightly longer, so if you use them with stock hangers it moves the axle back about a 1/2”. Probably a little less. If you read the description on the link you’ll see it says the hangers are shorter, not the springs.
It allows for a taller tire, and does the same thing all the drag race guys do when they shim their hangers back.
The zero arch spring is about the same height as stock. Maybe slightly lower, but not much.
Those have the same arch as my AFCO speings. I’m actually pretty sure it’s the same spring, just made by MP. But they’re made in Mexico. I run the Mopar XHD’s that also made in Mexico on my Challenger. They’re fine, but they sagged quite a bit after install.