To summarize, the trick is to have minor differences (mostly in tuning) between limited number of voices, and to keep those differences stable for those voices within a patch so that you simulate the differences between the 6 or 8 voice circuits in an old analogue poly?
This should be quite easy to do on the A4, if you load the same patch on all voices, slightly tweak them per voice and use the “use voice sound” poly setting. On a quick glance it looks like it’s mostly tuning but you can experiment with any parameters like ADSR settings. Super cool idea!