I remember one needs to purchase mindnode Mac and iOS apps separately.
Yes, that is why: you purchased rather than rented them, which was only possible separately.
They still provide that possibility as a legacy option but have indicated that it will be removed soon, just as they removed the upgrade options. They are shifting towards a subscription-only model. One subscription of $19.99 p.a. covers both platforms. Their Setapp version had to match that.