The key here lies in the combination of the two terms "national" and "socialism." "National Socialism" was in theory a different species than either taken separately, and it certainly opposed the sort of international socialism you mention, though it shared aspects of the popular socialism that had been current in Europe since the early 19th century. (In practice the state is best described as "authoritarian, but that is not your question.)

The idea behind Anton Drexler's "German Socialist Party" - the party a young Austrian corporal joined in 1919 - was not unique. In the days after the armistice that ended the Great War many a conservative theorist - discussed below - dreamed of a state that would both respect the national identity of the German community and provide its masses with an official alternative to Bolshevism. Their version of socialism did not have as its primary purpose the well-being of the worker but rather the harnessing of the worker's energies in the service of the German state/German volksgemeneischaft. German or nationalist socialism would concomitantly balance the working class's interest with that of the ruling class, especially the factory owner and entrepreneur.

The socialist revolution of 1918 encouraged some of these hopes, as when labor unions and industrial managers came together in November, 1918 to hammer out a compromise on wages, working hours and strikes. In the shadow of the Spartacist movement, here was a corporatist modus operandi where business magnate and worker could come together for the stabilization of society against the menace of Bolshevism. At a time when Bolshevik revolution appeared all but inevitable, such a nationalist-socialism seemed a practical way of staving off the worst. Walter Rathenau, the industralist-nationalist statesman who served as foreign minister until his assassination in 1922, exemplified the accommodation of nationalism-industry-socialism that many on the right hoped for.

Ernst Niekisch, Ernst Junger and Oswald Spengler all proposed nationalistic-socialist states that can be seen as theoretical forerunners of Nazism. Schematically: Niekisch proposed "National Bolshevism" as a dictatorship of the proletariat that would rise up to reject the internationalism of Moscow; Ernst Junger's "trench socialism" called upon the model community of soldiers as the basis of a new society by and for Germans; Oswald Spengler's 1919 Prussianism and Socialism rejected Marxist materialism in favor of a national community that would unify around traditional Prussian values. All three writers were influential on the right wing, and all three provided models of nationalistic socialism as a movement that would serve the needs of both worker and industrialist in a purely German context. There is, however, a certain zeitgeisty insincerity about these authors' use of the term. In 1918-1919, socialism was the future. To secure its place in the future these writers sensed that conservatism would have to take up the socialist mantle.

In this light Anton Drexler is little more than a political copycat. Without Hitler, his movement would have fizzled like so many others, including National Bolshevism. To give you an idea of how commonplace the idea of "German" or "National" socialism was, Hitler's party joined with Julius Streicher's in 1921. Streicher's Party? "German Socialist."

It's easy to see how these ideas became even more widely popular in the dark days of 1930-33. As the class conflict just barely suppressed by the various compromises of 1918-1921 began to bubble up again, many sought a radical solution. "National Socialism" promised to do away with the un-German Republic and its materialist socialism without sacrificing the worker. It was a pitch that by-and-large worked.

On that note, it's not quite right to say they were clearly "not socialist." There was a violent, revolutionary socialist wing to the movement in the SA, especially active in 1933-1934 under the leadership of Ernst Rohm. They did foresee a radical, socialist redistribution of property, amongst other things. In order to make his party more palatable, Hitler did away with Rohm and neutered the power of his SA in the 1934 Night of the Long Knives. After that, the "socialism" was little more than a label - though still fairly consistent with those earlier reactionary theorists of the post-war like Junger or Spengler.