That's a bit light on the details, and I can't make any sense of "To use stdio even you literally need kernel-level privileges on Console.", C's stdio doesn't require special privileges to use, nor does Crystal's, afaik.
More poignant examples would be `dirent.h` and `system`. Getting access to the file system on a console, or the ability to start up a process out of band is not allowed.
Maybe someone else will know that means.