Fat consumption does not necessarily equate to more stomach fat, unless you are eating excessive amounts. In fact, carbs and estrogen mimicking/endocrine disrupting components found in food, alcohol, pesticides, vinyls, personal care products, and plastics may be more responsible for fat accumulation around the abdomen. Not to mention reduced testosterone, which may be the primary cause of this.
As for muscles showing in skinny people, it is very unlikely unless they do ab exercises or have genetics for large ab muscles.
Something to consider in a food shortage situation: our bodies use fat and muscle to produce energy. Our body prefers using fat first, but some people will experience losing muscles more quickly than others. It is unlikely that you will see a starving person with abs, because muscle requires more energy to maintain. Your body will use up muscles as energy to keep your organs running. Hence the abs disappear.
Personally, I have been working on abs for 4-5 years and they took a long time to grow. When I eat significant calories or drink too much, they become less defined. When I cut out sugar, beer, and limit plastic/vinyl exposure they become more defined. Abs will usually be small and less noticeable unless they are trained intensely.
I also adopted a ketogenic diet consisting of (as calories) 70% fat, 20% protein, and 10% carbs. I noticed a more even distribution of fat on my body, my abs becoming a little more defined after a couple months. The fat was mostly, avocado, olive oil, cheese, and butter. The next time I do this I will cut out the butter and soft cheese, as it has more estrogen mimicking compounds.
We hear a lot about BPAs and BPFs so here is a less known problem...
Source about Phthalates: https://branchbasics.com/blog/2015/06/common-household-chemicals-phthalates-19-surprising-sources/