Nonetheless, EC 261/2004 requires that the carrier initiate a refund within 7 days of your request.

Don't stand on the form of whether KM emailed you about the cancellation. What you need is something which demonstrates that the carrier did cancel. A screenshot of your booking showing the cancellation or other demonstration that the flight was cancelled are more than sufficient.

For the benefit of others, there was nothing wrong with OP calling. But, a follow up email or webform submission confirming the conversation and reiterating the request, can be helpful.

No reason to delay seeking the chargeback.