No, you misunderstand the sentence. "Merely skilled" and "highly competent" are ways of describing level of ability. You might also say "novice/beginner", "expert", etc. "Merely skilled" means you are good at something. "Highly competent" means you are VERY good at something. That is what they mean when they say "differences". The difference in level between merely skilled and highly competent. "Sushi making" and "information design" are just fields, which is basically what you do for work. The word "disparate" is another way to say "different". Sushi making and information design are two very different fields. Differences between the "merely skilled" and the "highly competent" can be seen in fields as disparate as sushi making and information design. This means that there are big differences in skill level in all fields. Even in fields that are unlike each other, such as sushi making and information design.