Everyone can lose weight if they simply eat in a calorie deficit everyday. Do some people have a slower metabolism than other people of course they do. Women in general have a much slower metabolism than men because they are shorter and have less muscle compared to men but at the end of they day if they want to lose weight they still need to be in a calorie deficit everyday in order for them to do that.
Men who don't have visible abs are obese in my opinion and women who don't have a any ab definition at all are also obese in my opinion.
The genetics defects argument isn't even a valid argument because couples who are not related to each other can also have a child with massive birth defects.