I presume they're judging based on https://wiki.openstreetmap.org/wiki/Tag:usage%3Dmain: "a railway with higher traffic and higher speed than branch lines, often double tracked and/or electrified, may be high speed, usually longer than a branch line". I agree with davide that it's a travesty that it makes Sardinia look like a railway void at first glance. And semantically a branch railway needs a connecting main railway to branch off of. Surely the Cagliari – Golfo Aranci line at least deserves the main label: it's double-tracked from Cagliari to San Gavino, being electrified past that to Oristano on account of high enough traffic, and as the line's name indicates it officially continues even further for a total of 300 km.