The composer, YouTube user bd594, says what you hear is 100% pure, straight from the machines themselves—no effects or sampling were used. In the case of the Scanjet, the "vocals" were recorded in four separate parts using the one machine.
The aforementioned Atari 800XL is tickling the ivory on lead piano/organ, while a plucky Texas Instruments Ti-99/4a shreds on lead guitar. Rounding out the bunch are the floppy drive on bass and the hard drive clanging cymbals. As many have suggested already, the "instruments" could use a good tuning or ten.