Would this be proxy agency? Or is it different?

On the one hand, I think it's different. instrumentality seems to have a subtly different meaning from proxy agency.

But looking at their example sentence,

communicated through the agency of the ambassador , it is striking how similar these ideas/words/meanings are: the ambassador acted on behalf of his country (proxy agency: acted => agency; "on behalf of" => proxy); the communication occurred through his actions (= he was the means, he was instrumental in causing that communication to take place)

https://en.wikipedia.org/wiki/Agency_(sociology):

proxy agency is when an individual acts on behalf of someone else