Personally I'm not arguing on behalf of this outlook, because I think it gets semantical, but the general argument stems from observations made by Robert Sapolsky, who observed that all living beings are products comprised entirely of the environments they live in and the DNA they were born with, meaning that when a neuron fires within our brain as a reaction to stimulus, the course of its action is predetermined. 

From Sapolsky's perspective, for free will to be provable, then a neuron would need to fire in response to a stimulus in exactly the same way in a brain that is entirely lacking in the environmental and biological components that as it does in a brain which contains environmental and biological components. 

I wish I could remember the poster who mentioned this or the previous discussion on the topic, but that's my understanding of what Sapolsky argued on behalf of. I know there are numerous scientists and mathematicians who find that to be difficult to dispute, but my general take is that Sapolsky is a scientist who's saying free will can only exist if we have no evidence of why a neuron fires in a specific way, which feels anti-scientific, particularly since this is a test we cannot conduct. 

For that reason, we should treat these people far more compassionately and endeavour to provide them with supports to help them develop the necessary skills to adapt to a healthy, productive life. 

The predetermination is not whether or not he will drop the stapler or gently put it down.   Predetermination kicked in the moment the rich male student picked up something off the professor's desk to pose the challenge.

To reiterate my criticism of his outlook, I think like a lot of modern philosophy it hinges too much on the meaning or value we give to words. I recognize that I am the product of my DNA and environment, and that the origin of a singular thought can be measured (i.e., if you ask me a math problem, we can measure when and where the activity in my brain kicks in as I produce the answer). Therefore I suppose it would be theoretically possible for a sophisticated enough machine to scan my brain as the neurons fire and predict the outcome before I am able to react in real time. My criticism then hinges on the idea that the only way to prove there is no free will is by observing a "lack of evidence" relating to a situational response, which feels contradictory.