Sub-10% body fat is underwear model territory. At that low level of fat, every muscle is well-defined, no matter how small.

A lot of guys severely underestimate their body fat %. Sub 10% generally isn't possible without heavy dieting unless you're still going through puberty; genetics has nothing to do with it.

I'm aware of what it looks like, and I've been up at 12-15% before. It's absolutely genetic, most of my family runs very skinny. I have generally decent muscle definition, although the fat I have tends to sit in my hips and shoulders. My doctor suggested I might have less interstitial fat than most people, which explains a few other health effects.

I'm not bragging, there are serious downsides, but it's absolutely possible to sit at 10% year-round, just not while maintaining any serious level of elevated muscle mass.

and on top of that, it looks like there's other factors of diet than just choosing what to eat - it sounds like some people are just less hungry for indefinite reasons. genetics, gut biome, etc all probably play a part.