There may be export limitations for at least the first few weeks. Would not plan for 2019 deliveries for many, especially those outside US. Good luck to those preordering!
Supply restrictions are far more likely that export paperwork.
If Apple doesn't take orders from certain countries on 12/10 it is probably due to the demand bubble mismatch to supply. ( not particularly different than limited iPhone launches , though the supply squeeze cutoff at a different order of magnitude. Mac Pro in the multiple 1,000s range and the iPhone way, way higher. )
If Apple guessed substantially wrong on their preordering of parts ( e.g., too low on Vega II or too high on Vega II duo ) there will also be problems that they can only even start to correct once taking the pre-orders from the subset of countries they possibly start with.
Apple starts pre-orders on the 10th, but they don't ship until the 17-18th then there isn't going to be much time to build the customized versions for what Apple has not already made more than a few of in inventory. ( presuming they got the pre-order parts mix right months ago. ).