Plese educate me on the difference, as (at least in this context) they seem to be interchangable.

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To clarify, consider a hypothetical total autocracy, where the autocrat makes a decision, and everyone's behavior is altered by various means. By my reasoning, the autocrat has collected everyone's agency to themself.

Maybe it's not a perfect zero-sum, as you have inefficiencies in the system, but it does seem that you reach a point at which increasing one person's agency happens only at the expense of reducing others'

Your example of the autocrat who has a magical means to alter everyone's behavior is totally artificial and has no corollary in the real world, and therefore isn't relevant to the real world concept of agency.

Given that it is possible to alter people's behavior via non-magical means, I disagree that it's "totally artificial"

Well then specify the means, and make it a real example.