I think you mean that as a sovereign borrower, California can freely default or unilaterally impose a restructuring on its creditors. California is the debtor in this case. Federal bankruptcy courts wouldn't be able to interfere.

The wild card is what the California Supreme Court would do? While the state has sovereign immunity, in a default any creditors would certainly file suit in state court. There isn't much law covering the situation so it is conceivable that the judicial branch would order the executive and legislative branches to raise taxes or sell assets in order to meet obligations. That would create a constitutional crisis.