Be careful with the words here. The ICJ is the final decider about one specific definition of genocide. However, there is nothing that says that is the sole valid definition of genocide. In fact:

According to Ernesto Verdeja, associate professor of political science and peace studies at the University of Notre Dame, there are three ways to conceptualise genocide other than the legal definition: in academic social science, in international politics and policy, and in colloquial public usage.

• The academic social science approach does not require proof of intent,[11] and social scientists often define genocide more broadly.[12]
• The international politics and policy definition centres around prevention policy and intervention and may actually mean “large-scale violence against civilians” when used by governments and international organisations.
• Lastly, Verdeja says the way the general public colloquially uses “genocide” is usually “as a stand-in term for the greatest evils”.[11] This is supported by political scientist Kurt Mundorff who highlights how to the general public genocide is “simply mass murder carried out on a grand scale”.[13]

Source: https://en.wikipedia.org/wiki/Genocide_definitions