HN is not just read by IT professionals. And they might be a bit curious about Linux startup without necessarily recalling what they know or never knew about hexadecimals.
It's fine to explain things to people that are not into that topic. But I also wonder what the target audience is. On one hand, he explains what a hexadecimal ist, but then jumps over essential information needed to understand the boot process. It's like: "Learning to swim: This is water, it is wet, if you jump in you sink and in water you cannot breath. Then you start moving your body parts and then you swim."