Bicycles have had gears for almost a century, and they allow to tackle hilly areas easily. Also, the Netherlands is notoriously windy, and a headwind is just as difficult as a hill.

No, what makes the Netherlands different is their street design prioritizing safety rather than speed at all costs. When the streets feel safe from speeding drivers, more people choose to ride a bike.

Considering I'm not Dutch, you may feel reassured there is no superiority feeling at play here.

I agree with another commenter that while flat, the Netherlands have their own hurdles (biking with a strong headwind on the banks of the IJ is not easy, even if flat), and I definitely agree that their city design is what makes this unique.

I lived in various parts of France growing up, and I can assure you there are flat cities there, yet biking in them felt very risky at best.