I have an Aldi PAYG SIM, but unfortunately have no requirement and therefore no experience, of call diverts, but I would imagine that it should be possible.
I, also, don't personally know anyone else with Aldi mobile, so have never called to another user, free ......... which, I assume, is why they have a zero rate, in order to attract new customers. But as far as I'm aware, there is no catch - they're just promoting their own network (which uses Sunrise, btw).
best bet, is to call their infoline and ask.
https://www.aldi-mobile.ch/de/services/infoline.html