I think you're conflating a working title with a codename. A working title is exactly that; a project title that is meant to be temporary until they think of a better one, because they have to call it *something* while they're making the thing. This is fairly typical with large studio productions as usually the marketing people don't really get all that involved until post production and they can have a significant influence on the final title (but not always.) Indeed a production can go through several working titles over the course of production (pre & post) before then finally settle on a release title.
A codename is as one might surmise is a fake working title chosen to deliberately obfuscate exactly what the project is about, and as such is mostly a bookkeeping thing, used mostly just for locations shoots. Granted they're functionally identical in a sense, but the distinction is one of intent. The former exists because: "fuck it, we need to print something on the receipts!" the latter is because: "we'd rather not have the location shoot swarming with paparazzi & rubbernecks, nor have all the local businesses suddenly put an extra zero on the end of their prices because they think we're made of money."
Incidentally, the latter was the real reason for the now legendary 'Blue Harvest' codename for 'Revenge of the Jedi' (aka: the actual working title of the film.) It was no secret the film was being made. Indeed it was widely reported, which is kinda the point. Star Wars was a known quantity, and would have been recognised if the logo was plastered all over the crew gear and vehicles. It's only partially successful because the minute you start building a giant plywood spaceship out in the Yuma dunes, and Harrison Ford, Carrie Fisher & Mark Hamill roll into town, people will put it together. The point is usually to slow the spread of rumours not stop them (because you can't) so by the time the looky-loos and/or the press show up, you're hopefully almost done with the shoot and ready to move elsewhere.