Edit: to clarify, this is from the Wikipedia entry on left-wing politics:
In modern politics, the term Left typically applies to ideologies and movements to the left of classical liberalism, supporting some degree of democracy in the economic sphere. Today, ideologies such as social liberalism and social democracy are considered to be centre-left, while the Left is typically reserved for movements more critical of capitalism,[9] including the labour movement, socialism, anarchism, communism, Marxism and syndicalism, each of which rose to prominence in the 19th and 20th centuries.[10] In addition, the term left-wing has also been applied to a broad range of culturally liberal social movements,[11] including the civil rights movement, feminist movement, LGBT rights movement, abortion-rights movements, multiculturalism, anti-war movement and environmental movement[12][13] as well as a wide range of political parties.
So some classify liberals as center-left, but center leftists aren’t a thing, and calling them just leftists would be inaccurate.
Your citation says social democracy is a leftist ideology, that encompasses a significant number of American liberals no? I don't really think anyone would say Bernie is right-leaning, for example?
My source says it’s a center-left ideology. I would identify Bernie as center-left, based on that. But calling him a leftist would be inaccurate (unless we also call him a centrist). We could call him a center-leftist, but that feels clunky to me.
The poster is presumably "far left" - some flavor of socialist, communist, anarchist.
They use the term "liberal" in the classical sense which would include (US centric) Democrats and the so called "moderate" Republicans. Whereas in mainstream discourse "liberal" has come to mean "left" but really that's only left as far as is common in the anglosphere which isn't very far at all.