Hello, I tried to import the project in several Spine versions too, getting the same results at first.
If you downgrade Spine to v3.5.51you should be able to import the project correctly.
It was exported without nonessential data, so all the meshes may look weird once imported, having lost the internal edges, but if the source images don't have padding around them it may not be a problem.
That being said, make sure to have permission from the creators to reuse their assets. Some allow modding or the creation of fan animations, some might not appreciate it (: