The problem with providing a proper answer to this question is due to the fact that Nazism's ideological sources cannot be traced to a singular theory, world-view, or philosopher in particular. Nazism draws upon a variety of sources (German philosophers such as Schopenhauer and Nietzsche, French and Italian Fascists such as Giovanni Gentile, as well as German theorists at the time such as Karl Schmitt), and so we cannot clearly trace it's 'parent' ideology in the same way that you can trace, for example, the link between Karl Marx and Stalinism, or Islam and Wahhabism etc. Nazism at the time was not a formal doctrine and so allowed a wider degree of scope between 'theory' and 'practice' than the other examples given.

I'll try.

First. Stalin was of course a Communist. That he isn't, is a reduction of the argument. All large ideologies (from Liberalism to Fascism) come in many variations. The ideological thought of Stalinism is build from Marxist-Leninist thought. This form of Communism is strictly hierarchical (top-down vanguard party), authoritarian and builds upon a strong state. This isn't the only form of Communism. There are non-hierarchical, anti-authoritarian thoughts too. These ideas are nowadays the prevalent form of Socialism/ Communism. Stalin, of course, doesn't fulfill this, and people reject his ideas, therefore the distance themselves from this by labeling it something else. This argument is reductionist and also a political argument. The argument rightly stated would say that Stalin is not part of his or her branch of socialist thought.

As a side note: The question if stalinist thought is adhering to classical Marxist thought is again, a different question which isn't answered with the above paragraph. It is also more complicated.

Saying Hitler was no true Nazi, would be akin to saying Stalin was no Stalinist. A better comparison would be if Hitler was a "true" fascist. And this is actually something that gets argued. Some Neo-fascists in e.g. Italy (not only there, but there it is relatively prominent) argue that Nazism is different from Fascism and that Fascism doesn't necessarily lead to the excesses of the German Nazis.

Italian fascism is where Fascism first formed and has its ideological roots. Mussolinis fascism developed Antisemitism mostly in response to Germany, and at first wasn't especially antisemitic. Other fascist parties in Europe, for example the Iron Guad in Romania were pretty antisemitic.

National Socialism is a special form of fascism, and Fascism doesn't necessarily lead to the Holocaust. This is a defendable argument, but it is mostly brought forth by revisionists or Neofascists in defense of Fascism. But the argument is inherently flawed. While not every fascist party was deeply antisemitic, Fascism per se is racist and tries to attack and defeat everyone that is deemed a „threat“ to the nation. And this „threat“ is defined broadly. Oppression of the organized working class (Unions, parties), attack of minorities (e.g. Slovenians in fascist Italy), the supression of any dissident political voice (from liberals to socialists) are trademarks present in any fascist ideology. Furthermore fascism is by definition totalitarian and seeks to to soak up everyone and everything within the state. Everything must abide to further the Nationstate.

So in essence: You can argue that Hitler or National Socialism aren't per se fascist, but this doesn't absolve fascism from being an ideological thought that is built upon crime and oppression.

edit: I left out the difference between theory and practice. Just short, National socialism was very adaptable, what was right yesterday, doesn't necessarily mean that it is right tomorrow (see for example the SA). But I think it isn't important for the argument and necessary for the question.