Simple - you picked an arbitrary date to show that you should be higher on the Nic list. By picking a date that had no association with the merger, you wanted to stack the Nic deck in your favor by being considered active instead of furloughed.
Yes, every airline where the pilots are unionized that I know of uses DOH at that airline to determine seniority. But when merging carriers, you're by definition taking two company's seniority lists and merging them to arrive at the seniority list for the new company. The "gold standard", if one exists, is to use the negotiation, mediation, binding arbitration process to determine the seniority list for the merged company. That has been the standard method of arriving at a merged seniority list in nearly every deregulation era airline merger.
Jim