did you order something specific through costco, or do they broker between a local dealer that has something off the lot?
When we got our Cadillac via Costco, we were able to order the car to our specs. This was in, I dunno, late November when we did that (several years ago). The only push back we go was that the dealer mentioned the factory may not be able to build it due to scheduling or something. But in the end it went through.
I only mention that detail because when I ordered a Ford Fusion, or my Jeep GC, they simply took my order and that was that (the Fusion was ordered on a post-it note). So, that may be a GM thing.
Dunno how it might be different with a new launch high demand thing like the Bronco. But when I ordered my Jeep, it was a new launch of the platform, enough that folks were foregoing 2013s for the 2014s (when I went shopping my dealer flat out told me I should just come back later when the 2014s come out rather than get a 2013).
So, again, point being, the Jeep let me order a new car of a new launch with nary a peep. And I got employee pricing on it to boot.
But the Bronco is brand new, so, all I can think of is simply you'll may just be in line behind a large surge to get vehicles shipped to dealer lots, but dealer vehicles are not different from private vehicles. They have a work order, options list, are assembled by the same folks. Just need to get in line.
Whether Costco will be able to sell them out the gate is a different question.