Its not hidden. All the importers available when running the main KiCad project manager and pcbnew/eeschema launched from there are project-level importers where you provide the project file from the other ECAD software and it automatically imports the referenced schematics and boards. The importers available in the standalone pcbnew/eeschema programs are only importing the board/schematic, respectively, and not the entire project.
I realize the Altium importer is not feature complete. However, the menu item for it in pcbnew does not appear unless pcbnew is opened standalone, whereas in Eeschema it does appear regardless how launched.
For consistency, I think the menu item should always appear in pcbnew.