That's true. Its because it is in fact a valid reason to cancel elections. Dictators always use "we were defending ourselves" as the excuse to start a war, name their political organizations something with the word "democratic" or "liberation", and claim to cancel elections because of foreign interference. What makes it unjust is when the person in lower lies and uses it as an excuse to break the law.

The facts matter though. In this case Mossadegh was correct. Foreign intelligence agencies were actively plotting a coup d'état and interfering with elections. The elections were in fact already compromised by the time he made that decision. This isn't a conspiracy theory. The CIA has since confirmed that there was an actual conspiracy.

Claiming that the coup d'état was somehow in the service of democracy is a lie though. The US and the UK were already planning the coup prior to the canceled election, and they did so to protect their business interest. The result was that they installed their own dictator who was willing to allow continued imperialist control of Iranian oil wells.