I'm not sure if that answer works. The fact that Northern Gallic swordmen have more armour than their southern counterparts shows nothing to do with their quality of life. And besides which, Gaul doesn't extend to Italy, and the Northern bits of Italy are just as inhospitable as the rest of Gaul. The Celts were generally farmers anyways, the fact that warrior classes have differences shouldn't have to do with their main population.

My theory is that the team just has sources that describe the Southern Gauls as having less armour (They're bare-chested in EB) than the Northerners (Who seem to have chainmail underneath their shirt)