The first thing you need to understand is that both sides are telling you what they, politically, want to communicate to you. Everyone knows that the Soviet Union and Maoist China were bad. They may disagree on what about them is bad, or what specifically went wrong - but both the conservative and left-wing people in your life understand that these were authoritarian, and frequently violent, regimes whose connections to leftism can reflect poorly on leftists in the West.

In other words, the USSR and PRC are hot potatoes these people are trying to throw around. The right wing wants to connect the left to these regimes as much as is possible, in order to damage the image of their opposition. The left wing people you've met understand this and naturally oppose it, but they also understand the implications of being connected to authoritarian and even genocidal regimes, so they try and diminish any relationship their ideology may have with them.

The answer to your question is actually very simple - Yes, they were left-wing. The USSR and Maoist China were both regimes formed by local communist parties. Both parties existed, published and acted on a national scale decades before taking power. In pre-communist China, this is the Chinese Communist Party, and in the Russian Empire we are specifically talking about the Bolshevik faction of the RSDLP. In both cases, the people who were in leadership positions of the parties after the seizing of power had already been active members for decades.

Many who eventually wielded enormous personal power in the dictatorships, like Zhou Enlai, Mao, Stalin and Molotov (to name but a few) had long suffered and sacrificed much of their lives to the cause well-before success was in sight. Quite a few, famously Stalin, had suffered imprisonment in harsh conditions for their political activities. Mao's second wife, Yang Kahui, and their two sons were captured in 1930 by an anti-communist warlord. She was tortured and then murdered because of her connections to Mao. Mao and Yang's oldest son, Mao Anying, witnessed her death. In short - if these people were cynical opportunists, why would they have spent nearly all their adult lives being open communists when it was very dangerous to do so. Why would many of them have risked their lives, their families lives, fought in civil wars, murdered contemporaries, suffered torture and regularly gone to extreme action and risk for decades in some cases if they weren't true believers in a socialist revolution?

On top of this, we have a great deal of their personal publications in pre-revolutionary and pre-success communist literature. We have everything Stalin published before the Russian revolution. We can do the same for Mao and Lenin and many others. All of them were committed and passionate leftists who regularly referred to and argued about the work of philosophers and thinkers prominent in Leftist philosophy. In the policies they made after coming to power, they still referred to the pamphlets, essays and books they had read, written and debated before coming to power. Even as a dictator, Stalin still noted the thoughts of certain party members who had been more prominent writers than him before the revolution.

It is absolute consensus by academic historians and other related scholars (economists, IR theorists, etc) that these were left-wing parties and left-wing movements. The personal archives, lives, thoughts and feelings of Mao, Lenin and Stalin are understood to the degree that, now in 2018, there is almost no serious reason not to believe that all three men were committed leftists who firmly held left-wing ideological convictions informed by the philosophy of Marx, Engels and, of course, each other. The "opportunists" in either regimes upper echelons during the early years are very few compared to those who had serious communist and leftist credentials. In both the PRC and USSR the upper leadership in the revolutionary periods was socially tight-knit, many had known each other for decades and in many ways - had literally grown up together. This also meant that grudges and rivalries tended to run very deep - and sex scandals could cause dramatic political shifts. The opportunistic and 'skin-deep' leftists in both early regimes tended to be military men or other individuals who brought remarkably useful skills and assets, and thus whose poor political credentials were worth overlooking.

In total - the USSR and PRC were founded by leftists, and administered and controlled by leftists, their early policies were constructed in reference and response to leftist philosophy and ideology. These were inherently left-wing regimes from their inception - and it is grossly dishonest to pretend otherwise.

The debate becomes very complex and fierce however, after we move from the 'revolutionary generation'. The Chinese Communist Party now has drifted very far from Mao and Zhou Enlai, and is largely run with very different considerations, aims and intentions to the CCP of the 50s, 60s and 70s. The USSR after Stalin's death in 1953 is typically considered to have still been leftist, but also to have definitively become less of a 'radicalised' and revolutionary power.

Edit - there is a common error, much bemoaned on reddit, that occurs often in respect to Nazi Germany. In America, specifically, left and right wing distinctions also apply to the role of federal government in state, local and personal jurisdictions - with many conservatives being "small government". Erroneously, some people assume this American left-right distinction is applicable to all parts of the world and time periods, when it is actually almost entirely restricted to the contemporary US. This leads to the error that because the Nazis were "big government" they can't have been 'true' right wing. This is basically hogwash, obviously.

I wonder if the people you've met might believe a reverse error - that because the USSR and PRC didn't have policies or beliefs common in modern American leftism (Queer politics and radical feminism for instance) then surely they're not 'true' leftists. This is obviously, just as much of an error.

Edit - here are my sources

Alexander V. Pantsov, Stephen Levine, Mao: The Real Story. Simon&Schuster, 2013.

Benjamin Yang, Tony Saich, The Rise to Power of the Chinese Communist Party: Documents and Analysis. Routledge, 1995.

Laszlo Ladany, The Communist Party of China and Marxism 1921-1985. Hong Kong University Press, 1992.

Stephen Kotkin, Stalin: Paradoxes of Power, 1878-1928. Penguin Books, 2015.

Stephen Kotkin, Stalin: Waiting for Hitler, 1929-1941. Penguin Books, 2018.

Orlando Figes, A People's Tragedy: The Russian Revolution: 1891-1924. Penguin Books, 1998.

Are you asking about the presented ideologies of the people in charge or the actual policies?

It’s I think the MOST important thing to be aware of is the fact that “left” and “right” are amazingly nebulous terms, and calling something “leftist” means nothing if the person you’re talking to has a very different conception of “leftism” than you.