I've had a couple of the "poor man’s Heuer" and there is no doubt that they were assembled in the same factory from the same parts bins, by the same people. In fact the Heuer marks are milled out and Zodiac is stamped in the case backs, clear as day, just as clear as the over printed dials.
Like Jeff, Mark and others have said this is a rather straight forward thing, no real mystery. Mark already noted that the Swiss watch industry at the time was mostly horizontally integrated(I think by law); few made all of their own parts. Everything was farmed out to the most capable company. It was kind of forward thinking at the time. Most major industries are doing this nowadays, in fact you could make the argument that the Swiss watch industry as a whole these days is actually kind of going backwards by going to a vertically integrated business model.
In the end it all comes down to what you believe. If you think that a GMC Sierra pickup is different from a Chevy Silverado, well then I have a bridge to sell you.