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.
Not at all. I simply suspect that you are uninformed about why cycling is popular in the Netherlands. In the 60s the Netherlands was just as flat as it is today, but it wasn't a cycling paradise. It all changed with the campaign "Stop de Kindermoord" (literally translated as "Stop the Child Murder"), which began in 1972.
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.
It's completely flat and the obvious reason why everyone cycles. Nothing to do with mindset, like you're somehow superior to the rest of EU.