Ultimately, I don't have a good answer here, but I can provide what I have learned. It is possible to export two of the brushes, "2 inch blender" and "2 inch landscape brush" by right-clicking the Mystic Mountains Essentials category and selecting "?ExportCategory?" (I'm guessing the question marks aren't supposed to be there, but they are), which exports a ".brushcategory" file. It is also possible to export the aforementioned "2 inch" brushes via right-clicking on them, which provides ".brushvariant" files. These files can be imported in Corel Painter 2019 via the
Brushes / Import menu option. Unfortunately, the other brushes do not export.
Since I was unable to find any solutions online, or on my own, I contacted Corel technical support. After a few abortive attempts to explain that I had received the brush pack via a bundle download rather than form the store, which meant I couldn't just redownload it from the store, they first sent me a ".pkg" file (which seems to be a zip archive, but is apparently a Macintosh install file), which they had no idea how to import, and then the actual installation package, which basically runs as an executable.
So, long story short, as best I can tell, there is not a good export/import path between the two programs, but there is a side channel by which tech support can just send you the brush package in question (which you will want to store away because it will not appear among your store purchases).
As another note for those who missed their chance at the Mystic Mountains brushes, there's a very similar set of brushes in the Udemy course, Corel Painter 2018 Brush Set and Brush Tutorials, complete with explanations of how to use them and how to adjust their settings, and the creator of those brushes, Karen Bonaker, has them for sale on her site.