Earlier quoted context omitted.
On what the article claims: my understanding is that the sandbox restricts all non-user initiated actions. That is, if a user initiates it by clicking, dragging, OK-ing, etc., then all the old system interactions are still allowed. If you want to take a screenshot, of course you can so long as the user hits a keyboard combo to do it. Where it's a real hurdle, however, is for those programs who have to monitor and int…
There's APIs to interact with the calendar store; no calendar application should ever be accessing the calendar files directly—so apps like Fantastical are probably already OK. I haven't deeply examined the sandboxing rules and capabilities, but I think that this could improve application interoperability because they will need to work through APIs directly rather than through filesystem backdoors.
(Perhaps there is some other sandbox-friendly way to get calendar info ... I don't know.)