Yeah, that piece of advice by itself does sound weird. But I suspect that the point that he/she was trying to make was that you shouldn't just use your essays to provide a more verbose description of the same projects & accomplishments that you highlight in your resume. If you only did that, then of course your essays aren't adding much to your application.
I can easily think of examples where you would talk about some resume accomplishments in your essays, though. For example, one bullet point on your resume might read something like, "Saved the company $2 million per year by leading a cross-functional task force from idea to completion." That by itself is impressive, but it doesn't say anything about
how you got it done. Say if one of your essay questions is, "Describe a time when you achieved a goal despite opposition from others," you might want to talk about how someone in manufacuting strongly opposed your cost-savings ideas, and how you convinced that person that it was the right choice for the company.
That's just a generic example, but you see where I'm going. Your resume will be mostly just the "what" of what you accomplished. Your essays can provide a lot of the "why" and "how."
Also, the other point that the admission person may have been trying to make is that you don't want to solely write about professional accomplishments. Don't be shy about discussing your extracurricular activities and personal interests, especially when they show your passion for something.
Best,
Scott
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Co-Author, Your MBA Game Plan