I think what he is saying is that if he had created an array in C++, then tried to use that array with Niagara using a blueprint to set/access the data, it would fail.
If i’m understanding everything correctly, he had to either set the Niagara parameter using C++ or he had to create something in blueprint to store the data from the C++ array being accessed in blueprints and then copy over the data.
I’m not sure that this solves nor addresses the question of why the "Niagara Set Vector Array” doesn’t work the way its supposed to.