An expression used to illustrate the limitations of democracy and the law vis-à-vis practical and moral affairs. In the sense that Hitler's rise to power was accomplished in a democratic fashion, it is not wholly untrue. If "everything" is taken to mean "everything," however, it is patently ridiculous.
We all know that Hitler broke innumerable treaties and pieces of international law. He flouted the Treaty of Versailles by rearming Germany and re-occupying the Rhineland. He ignored the Munich Agreement, the Kellogg-Briand Pact outlawing war, non-aggression pacts with Poland and Russia—the list goes on. That doesn't even consider the concept of crimes against humanity, for which he takes the ever-lovin' cake.
Nor did Hitler respect the laws of Germany. Ever hear of the Beer Hall Putsch? Hitler was quite rightfully convicted of treason, and served time for it. After he got out and promised to stick to the straight and narrow, his paramilitary types did what you might expect paramilitary types to do.
But let's not split hairs. Hitler was legally appointed Chancellor, then consolidated power using the emergency powers the legislature had granted. And once a man takes absolute power, what is legal or not becomes an academic question.