For those having trouble importing the brushes on native installs in Linux, the author of this github issue traced the problem to the default chunk cache size of 8mb in libpng, preventing any brush larger than 8mb from importing since libpng will truncate the metadata when krita attempts to read from the .kpp file.
Applying the patch to increase the chunk cache size resolved the issue for me. But if your package manager does not support applying user patches, then copying over the resourcecache.sqlite of a compatible version of krita that was able to successfully import the brushes seems to work just fine too.