Leftism starts at Marxism. After Marx, social thinking, economic and political analysis, it cannot be what it was before. Marx is inescapable.

Whatever tendency one goes on from there, weather it be anarcho-communism, orthodox Marxism, Marxist Leninism, Maoism, third worldists, Trotskyism, democratic socialism, socialism with Chinese characteristics, Juche, etc, it's all on the left as long as it starts from Marx's critique of capitalism. That's what differentiates leftist anti capitalism from egalitarian/radical liberal anti capitalism.