They specifically phrased it as further left of the left, which in this context "left" clearly just means "an actual socialist who doesn't want billionaires to exist."

On that note I'd absolutely disagree that tankies are further left than a real socialist. One of the many reasons a single axis is meaningless for in depth political discussion.

Well I certainly agree that left-wing extremism is a thing, including among a variety of leftist ideologies.

My issue is that I don’t think tankies should be considered leftists at all. They are admittedly hard to place on the spectrum because their ideas grew out of leftism, meaning many of their ideals, language, and issues of focus are shared with the left. So to a casual observer they may appear to be leftists. But once these ideas have been completely twisted and transformed to defend and strengthen existing oppressive social structures, they share a lot more in practice with the political right than the left. A wolf is still a wolf even if it wears a sheep hide. And trading a king for a chairman doesn’t make you left-wing if the chairman has most of the same powers and no structures for ordinary people to wield collective power.

Left and right have always been about human freedom and autonomy opposed to the oppression of monarchy and similar institutions, but people have become confused because parties tend to shift to the right the more power they gain. Today people seem to view left and right more as competing sports teams than the broad ideological schools of thought they are. Tankies do not fit with leftist thought and should not be considered any form of leftist, regardless of how extreme they may be.