Yep. Once the NJ Supreme Court made their decision, that's it. I think the NJSC blew it, but SCOTUS is in no position to overturn.
The New Jersey Legislature, the Constitutionally-authorized state authority regarding the election of Senators, did NOT authorize the NJ judiciary to make this kind of exception. The sole role they gave the NJ judiciary with regard to ballot vacancy was to determine whether a candidate was qualified or not qualified in accordance with the law.
Had the statute regarding ballot vacancies read something like "...no less than 51 days prior to the election except in cases that the judiciary may see fit....", AND there was no federal law contradicting this with regard to Senate elections (Congress does have supremacy in this regard), then you would have a strong states' rights case.
SCOTUS only refused to hear the case. No precedent has been set, except that they may at times chose to ingnore state courts' failure to enforce their own laws.