That's for the automotive embedded version of Linux they created. The AMI isn't quite the same, but they have a kernel specifically patched and optimized to run in a kvm hypervisor, they're using systemd-boot, which is a bit faster than GRUB, which is likely the bootloader these other distros are using.
What else they did would probably require booting into Clear Linux myself and checking it out, but disabling services you don't need so systemd isn't wasting time, baking all kernel modules you know you'll need directly into the kernel instead of storing them as modules, and not using initramfs if you don't need it is probably the "general" answer for how to get a faster boot.
If you really want ultra-fast, you can turn Linux into a unikernel by compiling the kernel with your application as init, not using any other services at all, and booting via the EFISTUB instead of using a bootloader at all. I don't know if the AWS EC2 services provides any way of doing that, though. On your own machine, you can either use efibootmgr from a different booted system on the same BIOS, or by setting boot order in the BIOS, or naming the EFISTUB whatever that magic name is that EFI automatically loads if a boot order isn't configured in NVMEM.
Restructured the boot sequence and probably stripped everything which did not fit in a server / cloud context, also mayhaps removed everything which was not necessary from the base. Not unlike openbsd.
