Just from the top of my hat, the latest example of a hundred-plus years worth of statements/actions that disprove that statement: https://www.n-tv.de/mediathek/videos/politik/Linken-Mitglied...
Shown there, a statement caught on tape at a strategy-summit of the German far-left party Die Linke (I freely translate): "...regarding Energiewende [German Green New Deal]… after the revolution, when the 1% has been shot, we still want to heat our homes…"
To which the party chairman, Bernd Riexinger, present on the same stage replied: "We're not gonna shoot them, we're going to put them to good use"
Maybe I'm biased, but I come from an underpriviliged background (in a first world country), yet made great strides and climbed the social ladder. Throughout my experience, people the most concerned with trying to frame my poor childhood as a "class situation" were almost always middle-to-upper-class people.
At some point it even felt like rich kids use this framing to keep poorer ones form passing them on the social ladder. But I'm convinced by now that the urge to frame groups into instituitional victims, is something more personal: Having the luxury of never having to grow up because of their parent's financial background, they rather choose to pick a fight they can't loose because it's not theirs.
I'm also from an underprivileged background (in a first world country as well), I witnessed first hand how the odds stacked against me have translated into my lived experiences.
Maybe unlike you, I have fought and lost against people with better parental financial situation I have a perspective that this is an issue, not something that can just be gritted through. Not all success is through hard work.
If I were to be just cynical, I could argue that your loss may be due to processing your uphill battle via this whole marxist framing and not via a more independent "OK, how can I improve my situation?" (e.g. switch companies/sector/trade/town/country).
But I agree, Not all success is through hard work. . Absolutely. My success is a proof of this, because I'm a mediocre programmer at best. Which invigorates my disbelief in programmers advocating the ultra-left Marx: In the programmers job-market how can one feel disadvantaged at all? The financial crisis 2008 left us untouched, while the majority of society were furloughed or fired. Same story this year.
My advice: This odds-stacked-against-you-framing is your biggest waste of time. Financially and spiritually.
What I would call "ultra-left" are militant anarcho-communists who believe in abolishing strong systems of centralized control in favor of community rule. So no nations, only self-governed communities of some arbitrarily small size. This jives with "ultra-left" in areas with leftist militia uprisings. Not saying it's practical, but there are well-established schools of leftist thought at play that predate Stalin and Mao.
Liberalism is a center-right ideology that tries to marry right-wing capital control economies with left-wing social values. Many on the left regard liberalism (and neo-liberalism) as a failed project, because with any clash between economics and social values, the economics tend to win out. I'm not saying communism is the answer, but a move away from a scarcity-based economy would be a good start.