I only know what I've been reading and hearing over here but my impression is he was a subordinate in one of the earlier, socialist or all but socialist administrations and has only recently claimed to be no longer part of that mind set. Again, shades of US candidates.
Yes, he was part of the current government but he's an independent candidate not part of the socialist party (so officially not a socialist even if he might have some socialist views when it comes to some things).
Probably why he's so popular.
He's a centrist, not as left wing as Clinton but not as right wing as Trump I guess you could say.