Yes the environmental impact of an AI agent performing a given task is lower. However we will not simply replace every programmer with an agent: in the process we will use more agents exceeding the previous environmental impact of humans. This is the rebound effect [0].

Your reasoning could be effective if we bounded the computing resources usable by all AI in order to meet carbon reduction goals.

[0] https://en.wikipedia.org/wiki/Rebound_effect_(conservation)