With a planted cycle, the way I see it is that the plants 'take the edge off' from ammonia causing harm to fish.
Even in a properly cycled tank that has a filter packed with BB, you can't gaurentee that the fish don't come into contact with the ammonia that hasn't yet been sucked up by the filter and nitrified.
If you have enough plants that are thriving, then arguably they are quicker to absorb ammonia in the water than a filter would be. I say arguably, I haven't read any supporting research on this.
But do we know for example, that when the filter sucks up some water containing ammonia, that all of that ammonia is (a) converted to nitrite and (b) converted to nitrate in one pass?
Its all about balance too: plant mass vs ammonia producing livestock. Ideally plants would take care of all the ammonia produced. But I think that is rare for a lot of people who like to get as many fish in their tanks as possible. So as the general advice has been, add a couple of fish to begin with. Whilst the plants are doing most of the work, theres still the potential that the filter will suck up ammonia and start a small colony of BB.
The more fish you add, slowly, the more plant mass you'll have and also an increase in BB.
I don't think its an exact science because everyones setup is unique. That's why its important to keep testing our water during the early phase to gain an insight as to what is likely going on, and help you out with the balancing act.
All requires a lot of patience unfortunately lol