Because if you spend enough time on the internet, especially the Western side, you start to hear voices of people who oppose their governments and not enough from those who support it.
Even many who are supportive tend to say that their government is authoritarian.
I’m also a Southern Vietnamese here to answer your questions. Party members are still required to study Marxism-Leninism. You are actually tested in how well you know ML ideologies before your application to the party is approved. But deal to history, most of the core members of the politburo is from the North and the Northerner are typically more hard line ML while many Southern party members flirt with socdems and liberal ideology.
When he mentioned liberalism I think he mean the ideology and not the culture war in the West. Most people living in those socialist countries still have to struggle mostly with consequences of colonialism and capitalist imperialism and is less embroiled in identity politics like in the West. Vietnam and Cuba is still better than most of the US regarding LGBTQ issues and gender equality though.
o7