It's a fake BIOS/DOS bootscreen to parody the startup of the original DOS demo (the c64 version of the demo does something similar).

It says DOS3.3 as a joke, because Apple DOS3.3 was a real, long-running operating system for the Apple II that predates MSDOS.

The actual demo runs no operating system at all, it's a custom bootsector designed by qkumba that can load arbitrary tracks off of a raw Disk][ floppy and is much faster than DOS3.3 or ProDOS and also requires very little memory overhead.