It's not compression - there is metal-to-metal contact from coil to coil all along the housing.
(I supposed this is technically not true, as each coil is pressed against its neighbour there is some elastic flattening at the contact points due to the low degree of osculation, but this is small relative to the total length.)
It's flexion - the inner cable is smaller than the outer, and if you have a compound curve the housing will tend to straighten slightly during braking, taking up the clearance.
"Compressionless", as most have noted, is stiffer than coiled housing, so it feels different.

It's pretty easy to see this with coild housing on the run from a non-aero rear brake lever to a rear brake.
If the front section is not the correct length, when braking it will flop about beteen the lever and the first top-tube attachment of course, but even when correct you can see the housing change a small amount.