She claims she offered to tell the people around her not to buy them but the airline refused. The airline says they had agreed to do that but she decided to buy them all anyway. So it wasn’t about giving up private medical information (which wouldn’t make much sense anyway since she announced it to the world through this article). There’s also the fact that you’re very rarely “forced” to fly anywhere, especially in Europe. I just checked Google Maps and a flight from London to Düsseldorf is a little over an hour. There are train options for about 5 and half hours. That’s obviously a pretty big difference, but definitely workable if the allergy is really that severe. There’s also the fact she could’ve called ahead of time and asked about options instead of trying to strongarm the flight attendants into doing what she wanted.
And yeah, it wouldn’t be that difficult to not sell peanuts but that’s not the only thing people are allergic too. Unless they were going to not sell any snacks and not allow any outside food, it’d be impossible to prevent all allergens. That’s one of those things that the only possible solution is the person with the disorder to do their best to mitigate exposure, which means making sacrifices like taken a few trains for 6 hours instead of a convenient flight. There’s lots of people that due to various restrictions can’t always take the most convenient option.