a fly’s movements were complex to argue that a fly has internal agency

A fly has the ability to observe its surroundings and adjust its position in response to outside stimulus. A leaf does not. That, alone, adds a dimension of activity that the first possesses and the second doesn’t.

You can argue that the fly is still a deterministic agent, but the ability to observe and respond adds a dimension of activity that’s more complex than a leaf, which can only move based on the surrounding wind currents.