At https://www.top500.org/statistics/overtime/ click "Operating System" and "Performance Share" to see:

  - 300 PFlops: Linux
  - 260 PFlops: other 
  - 150 PFlops: Cray Linux
  - 130 PFlops: CentOS
  - 1 PFlop: bullx supercomputer suite (supports RHEL/SUSE/etc)

More correct title would be "All 500 of the world's top 500 supercomputers are running Linux kernel". Of course nobody uses off-the-shelve Ubuntu (or another distro) to power supercomputer. I'm not an expert, but I bet that it's heavily modified Linux kernel with custom userspace optimized for a given task in mind (scientific calculations, simulations etc.)