They would try to decide what intelligence in humans is really for, not merely how much of it there is.
这个句子的结构，其实 not 权当连词。not 和 merely 没有关系，merely 可以不用。并非合在一起构成并列连词。网友的句子跟下句是一样的结构：
I want to see her, not you.
区别是：网友句子的宾语都是从句，而这个例句的宾语是代词。

我也举个从句作宾语的例子：

I want to know how you did it, not (only/merely) what you did.