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!