David Lublink reported the following as an Ubuntu bug:

int
key_perm_ok(int fd, const char *filename)
{
[...]
          error("Permissions 0%3.3o for '%s' are too open.",
              (u_int)st.st_mode & 0777, filename);
          error("It is recommended that your private key files are NOT accessible by others.");
          error("This private key will be ignored.");
          return 0;
     }
     return 1;
}

The text "It is recommended that your private key files are NOT accessible by others." should read "It is not permitted....".

There is no workaround to use a non-protected private key, therefore it is incorrect to say "recommended".