I think you actually, without intending to, raise the reason why this is an exceptionally powerful point. Given the diversity of academic opinion on so many fundamental subjects, consensus on any topic is extraordinary.
I actually don't agree with you that "legal definitions" are as hotly debated or that the existence of debate in general negates consensus on specific topics. And I do think one important point with genocide scholarship is regarding muddying the waters with tom-ay-to/to-mah-to approach to definitions. Treating definitions as inherently transient is an important instrument in normalizing cultural acceptance of genocides when they're unfolding in real time, which is why that tactic is targeted by so much scholarly criticism.