The refusal is logically fallacious, and it misrepresents the Van der Lans case on which it relies. Let's pick it apart:

Misleading claim about the case

They claim that the Van der Lans decision holds that "a technical error which results in the replacement of a component can be considered to be within the airline's control and thus give the right to compensation." This mischaracterizes the decision. Actually, the ruling was

a technical problem, such as that at issue in the main proceedings, which occurred unexpectedly, which is not attributable to poor maintenance and which was also not detected during routine maintenance checks, does not fall within the definition of ‘extraordinary circumstances’ within the meaning of that provision.

The ruling describes the technical problem "such as that in the main proceedings," which leaves it open to interpretation -- and further litigation -- as to which aspects of the technical problem could distinguish this case from others. But the airline asserts that the ruling applies only to technical problems that result in the replacement of a component. That does not appear to be critical to the court's reasoning in this case, except that the fact that the replaced components having failed early was part of KLM's argument. The court held that this did not matter.

On the contrary, the court made reference to Wallentin-Hermann (C‑549/07, EU:C:2008:771), in which the ruling held that

1. [...] a technical problem in an aircraft which leads to the cancellation of a flight is not covered by the concept of ‘extraordinary circumstances’ ..., unless that problem stems from events which, by their nature or origin, are not inherent in the normal exercise of the activity of the air carrier concerned and are beyond its actual control. [...]

I would argue that the distinguishing factors in Van der Lans are actually those mentioned in the ruling itself. That is, the ruling applies to technical problems that are

1. unexpected,
2. not attributable to poor maintenance, and
3. not detected during routine maintenance.

Wallentin-Hermann, by the way, is where the airline gets the concept of "within its control," which isn't found in the Van der Lan ruling. But the test there actually has two parts, and they only mention one. For something to be "extraordinary circumstances" it must not only be beyond the airline's control but also outside the "normal exercise of activity."

Poor logic

Let us for the sake of argument assume that the assertion about the Van der Lans case is correct. That is, because this incident doesn't concern an actual technical problem, but only the suspicion of a problem, Van der Lans does not apply.

In that case, the logic is this:

1. Van der Lans says that a certain category of technical problem warrants compensation.
2. There wasn't actually a technical problem here, or the problem is of a different category, therefore Van der Lans does not apply.
3. Therefore, this does not warrant compensation.

The last does not follow. It's equivalent to this line of reasoning:

1. People with US passports are US citizens.
2. My next-door neighbor does not have a US passport.
3. Therefore, my next-door neighbor is not a US citizen.

But she is a US citizen. The fact that someone does not have a US passport actually tells us nothing about her citizenship. More generally, if the truth of A implies that B is true, that does not in turn mean that A being false implies that B is false. Wikipedia has an article about this where it is called denying the antecedent.

The last sentence is particularly egregious:

there was no technical fault and no components were replaced The Van der Lans decision is therefore not applicable

Okay, even if the Van der Lans decision really is not applicable, it still does not mean that the airline isn't responsible. The Van der Lans decision is not the only route by which responsibility may arise.

The inspection itself was within the airline's control or inherent in the normal exercise of its activity, so the "extraordinary circumstances" exception does not apply.

Conclusion

I would write something like this:

The reliance on the Van der Lans decision is misplaced. Even if were true that this incident falls outside the scope of that decision, which I dispute, that does not imply that the incident qualifies for the "extraordinary circumstances" exception. When the court identified one category of technical problem as not qualifying for that exception, it did not thereby cause every other kind of technical delay to qualify for it.

Even if this incident can be distinguished from the one in Van der Lans, the same logic applies. Van der Lans relies on Wallentin-Hermann, which says that the "extraordinary circumstances" exception does not apply unless a problem "stems from events which, by their nature or origin, are not inherent in the normal exercise of the activity of the air carrier concerned and are beyond its actual control." In this case, the delay was the result of an "inspection of the aircraft following a possible technical fault," which is certainly part of the normal exercise of an airline's activity.

Van der Lans underlines that "functioning of aircraft inevitably gives rise to technical problems" (paragraph 37) and that fixing technical problems to ensure proper functioning of the aircraft is part of the normal activity of an airline (paragraph 43). The same must be true of investigating suspected technical problems, for that too is "required to ensure the maintenance and proper functioning of the aircraft it operates for the purposes of its business" (paragraph 43).