Nihilism is useless. We've made enormous progress since the first human stepped on this planet so I would say we've disproven nihilism for good. Modern governments are definitely not purely tools for the owner class to enrich themselves.
Those economic theories are not proper economic theories. As in, mainstream economists don't support them. That theory is propaganda in service of some specific political parties.
> I don't see the contradiction between the two propositions "government is for the ruling class" and "there have been some progress".
"There has been some progress" is a gross understatement considering the age we're living in. There has been tremendous progress and the lifestyle difference between the poorest members of a developed country and the richest ones is much, much smaller than that between the poorest and richest members of a country 200+ years ago.