They are incorrect. Dell does not provide accidental damage coverage for a keyboard. The PC accidental damage coverage has to be purchased at the original point of sale. Dell does not replace keys on a keyboard. If you purchased the keyboard from Dell and not a 3rd party seller (example Microcenter, Newegg, etc.), after you have troubleshot with the DELL-Cares agent and they have confimed that the keyboard is faulty and still under warranty, Dell would exchange it.
I also have the same problem my Alienware AW510K. Dell tech Support told me that it is not an ongoing issue with the keyboard and that my keyboard replacement is not covered. So I needed to purchase an accidental damage coverage in order for them to replace it I’ve only had it for 4 months and is still under warranty. But they keep telling me that that is not an internal issue with the keyboard. To purchase the coverage. 
AW510K, key replacement