No. A "system disk" is simply any disk which the computer can boot from and has an operating system installed on it.
In most modern computer systems, the hard disk is normally the system disk. However most systems can also boot from a floppy disk, a cdrom, or even a USB thumb drive, providing of course that the media in question has the necessary system files on it.
Many older systems did not have the ability to boot from the cdrom drive or USB drives. On these systems the only options were booting from the hard disk or floppy disk, so if the OS hadnt been installed to the hard disk yet (or it was broken) the only other option was the floppy disk.