Hi davide84,
in my opinion, if you look only at Sardinia, then yes, the Cagliari–Olbia can be seen as the island’s main line. But if you compare it to the whole Italian network (or even worldwide), I wouldn’t really classify it as a true main line.

In Italy we usually try to follow RFI’s classification as a reference (with some exceptions), but it’s still difficult to decide objectively which lines should be main and which should be branch, while keeping criteria consistent on a global scale. I also haven’t yet checked how other countries have applied this classification in OSM.

Maybe it would be a good idea to refine the wiki definition, since like already specified the classification depends not only on infrastructure but also on traffic. Sardinia is a special case because it’s an island, but I don’t think it should be tagged as main just for semantic or rendering reasons.