There's nothing wrong with digital AA filter today, but they weren't as good or as cheap computationally in 1982 when the Juno was released. Most digital audio

There's nothing wrong with digital AA filter today, but they weren't as good or as cheap computationally in 1982 when the Juno was released. Most digital audio stuff used analog AA filters back then. Also, the Juno was polyphonic, so handling 6 sawtooths at the same time plus antialiasing would be too hard on the poor chip! And aliasing does happens in digital oscillators when you're using a fixed clock and sample rate: If you have flexible clock (per-voice) you can just increment the output value in equal steps (and then set it to zero at the end of the "sawtooth") and you won't get aliasing. But if you have a fixed clock, you have to use a (very simple) equation to calculate the position of the soundwave at a given sample number. So you get aliasing when you approach Nyquist frequency, and you need the anti-aliasing filter to fix it. And since it was 1982 the sampling rate of the chip was probably not that high, so you would get aliasing much earlier than at ~22kHz.