I am not sure how you would be so certain that they didn't? Is that based on the design director's video discussing the alloy? Because the two are not mutually exclusive. The alloy can, and very likely DOES, contain all of the elements he discussed in the video along with the mixture outlined in the patent. Apple has been very upfront that the watchcase is a gold alloy mix. (Wouldn't it have to be? If you think about watchcases that are truly solid gold, they are extremely soft and not very protective of the mechanical workings of the watch. Pieces like that are heirloom pieces that are only worn on special occasions, not every day and certainly would not be acceptable to house state of the art computer technology that is meant to be actively used, pushed on, used against scanners, and the like). Of course you are right that gold is heavier than stainless steel. But silver is also heavier than stainless steel. And copper is heavier than both silver and stainless steel. All of those elements were discussed in the design director's video of the gold alloy used for the gold watchcase. All that being the case, it would be wrong to extrapolate that the weight difference in the case is solely attributable to high volume gold content as opposed to what Apple has already stated, that it is a dense alloy mix. Finally, you can always use your eyes. If you collect 18K rose gold, as I do, then you know that it is a soft gold appearance. That makes sense because 18K gold is a softer metal than 14K gold. It contains some copper to create the rose gold appearance but the greatest volume is gold, which as you already noted is yellow. The watchcase of the "rose gold" is a beautiful but coppery color that in my view comes from a good deal of copper, and it is a lot shinier perhaps due to the use of palladium (discussed in the video). And as I already mentioned, both copper and gold are heavier than stainless steel. And palladium is three times heavier than stainless by some estimates. So again, it's incorrect to extrapolate that the weight is due to a lot of gold.
At the end of the day, the Edition is by far the nicest of the time pieces, but the price structure seems a bit off to me, (and to others, for example the Harvard Business Review issued a pretty good analysis on the price flaw in the Edition) given that the value of the watch lies in the technology and the technology across all of the time pieces remains constant. One is not paying $10K-$17K for solid 18K gold as that term is normally meant and understood. That is the bottom line and so in my view the price structure ought to have reflected that. A watch in the $5K range would have been more reasonable I think. I am very excited about my watch, but I have already reached out to some experts about getting the rhodium plating put on it to protect it from scratching. I wish Apple had done this for me. The watch I purchased is coming at close to $1000 after taxes, AppleCare (which might be voided after I protect the watch) and the extra band I bought. So it would have been nice for them to take that extra step and protect the housing with something scratch resistant since they chose to go with a high-sheen housing as opposed to brushed stainless steel which does not have these scratch issues.
Anyway, those are just my thoughts.