As to why the other side of the island is more ‘successful’ that probably has more to do with the huge debt that Haiti has accumulated that western countries insist they pay back

Now the significance you put on that pact I guess has to do with whether you believe the devil is real or not.

But it is one of Haiti’s founding myths.

http://muse.jhu.edu/login?uri=/journals/small_axe/v009/9.2laroche.html

According to Haitian national history, the revolutionary war was launched on the eve of a religious ceremony at a place in the north called Bwa Kayiman (Bois Caiman, in French). At that ceremony on August 14, 1791, an African slave named Boukman sacrificed a pig, and both Kongo and Creole spirits descended to possess the bodies of the participants, encouraging them and fortifying them for the upcoming revolutionary war. Despite deep ambivalence on the part of intellectuals, Catholics, and the moneyed classes, Vodou has always been linked with militarism and the war of independence and, through it, the pride of national sovereignty.

So, yeah if there is a devil, Haiti made a pact with it. Might explain why even though Haiti and the Dominican Republic share the same island, the Dominican Republic has been far more successful.