> If this is true, then wouldn't we just be biomechanical robots operating on stimuli

Yes, that's right. Which is one of the big problems with this line of argument for many people -- how do you reconcile "biomechanical robot" with the concepts of "free will" and "moral decision-making"?

I'm personally agnostic on the issue (I don't believe science is able to answer this question yet -- or possibly ever.)

I do think it's interesting that you say "just a biomechanical robot." The "just" implies that being a robot -- a fancy rock -- in some way isn't enough. But in my mind, there's absolutely no (objective) reason to think of a human as any better or more important than a robot, or a rock.