I believe he has to win gold at the World Championships to qualify or become the best all-arounder in Netherlands and place high enough to qualify (around 40th place among all competitors, I believe, will be enough) or win the most points at the World Cup circuit in one apparatus or the Netherlands must place on the podium in the all-around at the 2020 European Championships. The message is clear: if you want to go to the Olympics, you either have to be the best of the best in one apparatus, or at least be a decent all-arounder. Zonderland is definitely not an all-arounder, so he has to be the best either at the World Cup or at the World Championships. I'm not too happy with this either, but I believe this issue should not be pegged on FIG because it was a choice of the Dutch program not to prepare him to be an all-arounder, despite all the signs in the last 15 years pointing out to FIG favoring all-arounders over specialists. Besides, the Olympic quotas favor the best and most complete programs, not the best gymnast in one apparatus. Yes, not many people like it, but programs affected by this have their coaches to blame as much as they want to blame FIG.
About gymnasts not competing at Worlds, yes, this is true. Gymnasts from nations that hope to qualify a full team deliberately skipped the World Championships because if you compete at Worlds, you can only qualify for the Olympics if your team qualifies, or if you are the best all-arounder in your nation. Gymnasts who did not compete at the World Championships can compete at World Cup events and try to qualify as the best athlete in one apparatus, so even if the nation has qualified a team, up to two extra gymnasts can qualify at the World Cup. I can see only China, Japan, Russia and the US benefiting from this, actually. Yes, this is stupid and unnecessary.