Is Eurowings's EC261 compensation refusal accurate?

Background: was due to fly PRN-DUS-MAN, but 3 days before departure, the PRN-DUS route was reduced for July, running on Wednesdays and Saturdays rather than daily.

My immediate view is that such a service reduction is invariably a voluntary business decision, i.e. within the airline's control.

I now got a refusal not providing any clear details (original in German HERE):

We thank you for your message and apologise for the inconvenience.

We regret that we couldn't operate your flight EW6604 as planned. The cause of this was extraordinary circumstances which we couldn't avoid despite careful planning.

The legal regulations do not prescribe compensation when a so-called extraordinary circumstance applies. According to the European Court of Justice (ECJ), this term refers to any and all circumstances ultimately derived from events which, based on their nature, cause or impact, are not part of regular operations of the concerned airline and are outside of their actual sphere of influence (ECJ C-549/07). In particular, this includes external damages (e.g. bird strikes or foreign objects on the runway), strikes, severe weather, as well as all kinds of airspace restrictions due to governmental directives.

Per the view of the [German] Federal Court of Justice (BGH), this further includes all disruptions not immediately affecting the concerned flight, but indirectly having an effect on your flight (e.g. BGH X ZR 104/13 or X ZR 121/13). It is possible, such as in your case, for various external factors during the day to add up. Such a chain could cause further inevitable disruptions and eventually also affect subsequent flights.

This year too, regrettably, the staffing conditions of the different European air navigation services have been very tense. Therefore, delays could occur during the coordination of flight movements, especially through multiple airspaces. Such directives from flight control authorities, airport authorities or other governmental bodies must be followed, whereby any caused delays, as per case law, are to be classified as an external intervention in the planned flight operations, and therefore outside the scope of influence of the concerned airline (e.g. BGH X ZR 115/12).

The flight schedule of an airline is comparable to a clockwork, which only works if all cogwheels clutch together. If an impairment occurs during such a sensitive process, a delay or even cancellation of a flight is most often inevitable.

In order to reduce the impact on our guests as much as possible, we naturally arrange replacement aircraft to reduce delays or prevent cancellations. Unfortunately, in your case it wasn’t possible for us to take further reasonable measures and operate your flight as planned.

We’ll gladly examine the possibility of reimbursement as per the right of care (e.g. catering during the waiting time at the airport). You may send us the corresponding receipts as a reply to our message for review.

We hope to be able to welcome you onboard soon again in spite of the inconveniences.

My question is: what errors, if any, can I point to? I want to give them one last chance before taking the matter to the Kosovan civil aviation authority.

Note: again, it wasn't a cancellation on a single date nor a suspension of the route, but a service reduction for all of July.

UPDATE 2020-11-26: just received the compensation on my bank account today- Took almost 5 months from the disruption taking place, but well worth it!