In the United States, the German Nazi movement was associated with left-wing politics because Hitler admired American progressive eugenicists and environmentalists. The American left produced policy and propaganda for the Third Reich and pioneered the eugenics movement of the 20th century. It was in Nazi Germany where these ideas were put into action.
William Henry Fairfield Osborn was the founder of the American Eugenics Society, Head of the New York Zoological Society, on the board of trustees of the American Museum of Natural History, and one of the early collaborators of the Sierra Club. He and a man named Madison Grant published a report called The Passing of the Great Race detailing the threat to white race and national identity in the US from Native Americans, African Americans, immigrants, Jews, and Muslims.
He worked alongside John Muir to establish National Park designations to round Native Americans off their land and confine them to reservations. He and John Muir believed that Native Americans and immigrants were destroying the land.
The Passing of the Great Race influenced Hitler so much, passages of the book were entered in trials as evidence at Nuremberg.
See Redwoods and Hitler: the link between nature conservation and the eugenics movement
Other American left-wing progressives also influenced Hitler. David Star Jordan was one of the early leaders of the Sierra Club and was very influential in the eugenics movement. He advocated for forced-sterilization programs that deprived tens of thousands of women of their right to bear children.
He is also a cofounder of the Human Betterment Foundation, who developed Nazi Germany’s eugenics legislation.
See Pulling Down Our Monuments
Also see Eugenics on the Farm: David Starr Jordan
The article you posted partially answered your question.
Hitler wanted to unite the left and the right to eliminate, what he believed to be, a threat to the existence of the Aryan race and National Identity by wealthy Jewish capitalists. He conglomerated the ideas of the Bolshevist left and the Bourgeois right and came up with National Socialism.
In the article you posted, talking about the right, Hitler wrote:
They ought day by day to din into the ears of the masses: 'We want to
bury all the petty differences and to bring out into the light the big
things, the things we have in common which bind us to one another.
That should weld and fuse together those who have still a German heart
and a love for their people in the fight against the common hereditary
foe of all Aryans. How afterward we divide up this State, friends - we
have no wish to dispute over that! The form of a State results from
the essential character of a people, results from necessities which
are so elementary and powerful that in time every individual will
realize them without any disputation when once all Germany is united
and free.
The article you posted also shows that Hitler followed socialist ideas and believed that Jews were wealthy capitalists who falsified the social idea and turned it into Marxism.
Again, talking about the Right, Hitler wrote:
And finally they all fail to understand that we must on principle free
ourselves from any class standpoint. It is of course very easy to call
out to those on the Left, 'You must not be proletarians, leave your
class-madness,' while you yourselves continue to call yourself
'bourgeois.' They should learn that in a single State there is only
one supreme citizen-right, one supreme citizen-honor, and that is the
right and the honor of honest work. They should further learn that the
social idea must be the essential foundation for any State, otherwise
no State can permanently endure.
In the article you posted, Hitler later defined the "social idea" as the first of several "fundamental principles" that must guide the Third Reich.
"1. 'National' and 'social' are two identical conceptions. It was only
the Jew who succeeded, through falsifying the social idea and turning
it into Marxism, not only in divorcing the social idea from the
national, but in actually representing them as utterly contradictory.
That aim he has in fact achieved. At the founding of this Movement we
formed the decision that we would give expression to this idea of ours
of the identity of the two conceptions: despite all warnings, on the
basis of what we had come to believe, on the basis of the sincerity of
our will, we christened it 'National Socialist.' We said to ourselves
that to be 'national' means above everything to act with a boundless
and all-embracing love for the people and, if necessary, eve to die
for it. And similarly to be 'social' means so to build up the State
and the community of the people that every individual acts in the
interest of the community of the people and must be to such an extent
convinced of the goodness, of the honorable straightforwardness of
this community of the people as to be ready to die for it.
