It doesn't garuntee earlier German unification just early Prussian supremacy within the germanies
Nothing gurantees German unification, but that was the general trend. My point is that absent a war with Austria the minor German states are more likely to fall in line and not be abolished by Prussia.
More specifically:
OTL, after Frederick William IV rejected the "Crown from the gutter", the national assembly in Frankfurt was dispersed and Prussian troops intevened to put down uprisings in various minor German States. Prussia tcorraled most of the minor German states into the Erfurt union, in which a three class based electorate would elect a new Reichstag. The elections, however, were largely boycotted, and Saxony and Hannover bolted with Austrian encouragement. When an uprising in Hesse required intervention Austria weighed in and revived the confederation, and Prussia backed down.
TTL, assuming the Austro-Russian war is contained and that the war with Denmark dies down as OTL and the Fredrick William rejects the German Crown one of thee primary scenarios could wind out:
1. Prussia moves ahead with Erfurt Union plans. Saxony and Hannover don't bolt, as they lack Austrian backing. The electorate holds its nose and votes. A new German Union is born, but with weak central powers and less popular legitimacy and appeal than OTL. Austria is, for now, excluded. I view this as being most likely
2. Prussia moves ahead with Erfurt Union plans. Saxony and Hannover and others (Bavaria) seek an external backer and find one in Napoleon III (most likely). Prussia denounces the traitors to the German nation and occupies the dissenting kingdoms. Napoleon invades in support. Early Franco-Prussian war ensues.
3. Prussia moves ahead with Erfurt Union plans. Saxony and Hannover and others (Bavaria) seek an external backer and find one in Napoleon III (most likely). Prussia backs down. German confederation reformed, probaly with Austria included.