You state that your journey was due to commence on Friday, 23 April, but by this time European airspace had been re-opened. This means that it is possible that the flight(s) you were booked to travel on operated as planned.

If these flights did in fact operate, then you would have had to follow the advice you have pasted above, and ring KLM Reservations in advance to tell them that you did not wish to travel.

It sounds as if your flights did operate, but that you did not cancel your reservations before the departure date. Requesting a refund afterwards would then likely only result in the refundable portions of the taxes being returned to you. Could this be what happened?