The list is ALL addons purchased through the marketplace. There are no exceptions. The only time you don’t get cross-play between PC and Xbox is if you buy the Steam version. Then NO addons purchased via your MS account are transferred.
As long as the 3rd party dealer makes their add-ons available on both platforms, it will be available on both with 1 license. Not all 3rd party devs will make an Xbox version though. There are some devs that will only make their wares available on PC.
That’s less to do with licensing issues than it is to do with having to simplify their PC version in order to make it run on the more limited Xbox resources. Not every 3rd party dev will want to do that. Some will choose to release on PC only. But if they also offer an Xbox version and you buy it in the marketplace, it will be available on both platforms.
So until PMDG actually say whether or not they’ll be making it available on Xbox, no one knows the answer to that question.