How are you defining these terms? I've got response as "a reaction to a stimulus" and spontaneous as "uncontrived and unplanned".
To get you up to speed on the last thirty years of this argument, there are basically a bunch of people who don't believe that Osensei actually spent much time at all making his techniques work. He spent that time training in more esoteric modalities, and these gave him certain skills which allow his techniques to work much better than they could if he had spent time training techniques.
I personally advocate kata training, but I think the point of kata based training in Aiki arts is to give you a vehicle by which you develop the special skills without thinking about it.
Spontaneous is defined by Merriam Webster as "performed or occurring as a result of a sudden inner impulse or inclination and without premeditation or external stimulus." So as it relates to Aikido, what are you harmonizing with, when external stimulus is absent? This is a very esoteric subject and is fraught with misunderstanding.
I'm not a person who's inclined to disbelieve something based on a lack of evidence, when it's obviously inferred. We can't measure the animating spirit of organisms, but we can tell when it's there and when it's not. We can't measure Ki, either. But how can we measure it's effect, so we can know if developing it matters?