That's also my understanding - much of the swelling is caused by heat travelling up the filament itself. The PLA conducts heat, and even more so when it has absorbed moisture (unlike ABS, PLA is hygroscopic). I'm setting up a container with reusable silica gel bags to keep my PLA in and will give it a try in another week or so.
Still, cooling the aluminum block makes sense. Part of the design philosophy behind Magma is that stainless steel tubing is a poor conductor of heat, so wrapping it in a more heat-conducting material like aluminum or copper foil could actually make things worse.
The idea is that any heat which makes it up the thin-walled stainless steel tubing should be absorbed by the aluminum heat block, and the fan blowing on the heat block cools it by a few degrees. That small gradient is all that's needed to bleed off whatever heat makes it up the tubing. Unfortunately it doesn't help much with heat that has travelled up the filament itself, but I'm hoping that keeping the filament dry (using desiccant) will reduce the ability of PLA to conduct heat.