While I think it's correct to exclude people who are not fit (or visibly in the process of getting fit) when taking advice the assumption that whatever works for someone else will work for you is quite nuanced.

In principle, all other species eat their appropriate diets and there are no species where individuals eat vastly different diet. (e.g. no lions eat plants, no cows hunt for meat) Thus it's very likely it should be true for humans as well. However some differences definitely are present. The most famous example being lactose tolerance.

Thus if you observe that someone eats carbs and feels fine it may be that the person has higher carb tolerance. You may have high tolerance too or not but don't know for sure until you try. Though if you feel amazing and very happy and don't have symptoms of illnes or aging it's very likely you're fine.

Note that I specifically mentioned aging because some symptims that are commonly attributed to aging can be reversed with lifestyle change which suggests they actually aren't symptoms of aging.

Either way, good luck if you ever decide to try it out!