Completely different extension but this one has installation instructions that have different paths for installation depending on if its flatpak
If Inkscape was installed on Linux from Flatpakthe path is more likely ~/.var/app/org.inkscape.Inkscape/config/inkscape/extensions/.
https://github.com/lifelike/hexmapextension
edit - theres an open issue talking about this, too
https://github.com/inkstitch/inkstitch/issues/1712
Maybe these steps could help? https://github.com/inkstitch/inkstitch/issues/1712#issuecomment-1751680252