I don’t think your assessment of the Luna-class and the Constitution III is accurate. They are entirely different, and the PIC production crew deciding that “refit” is basically just a word that the Starfleet Corps of Engineers will drop at a hat doesn’t change that.
The instagram log that explains the history of the Titan A claims that it was constructed “using much of the internal components” of the Luna-class ship, and that’s why it’s a refit, but that plans to build the new ship on the spaceframe of the old were set aside mid-way through.