Can you clarify which row number you actually sat in during the flight? On the off chance that you were seated in a different row, but having the same charge for those seats, you wouldn't be entitled to a refund. (I know it doesn't sound like you were seated in a paid row, but we have to consider and eliminate all possibilities first)

It's probably not the reason for the confusion here, but it might help if you adopted the exact "names" that KLM use. You mention "Extra Legroom Seats", but that is the name used by KLM for the exit row 30. Kids can't be seated in the exit row, and KLM would never have sold you seats for the exit row - so if you keep insisting that you paid for "Extra Legroom Seats" - which KLM interprets as being exit row - then I can see why confusion might arise.

You purchased seats in row 17, which is the first row of "Economy Comfort". You should never call these seats "Extra Legroom" or anything other than "Economy Comfort".

It might help if you rang KLM, explained that you paid for Economy Comfort, got seated elsewhere, and have been refused a refund via the online form for reasons that make no sense. Make sure you have your receipt for payment of your Economy Comfort seats to hand.