NIPs aren't written for humans, they're written for whoever ships the client and the relay. From the repo's own criteria: a NIP should be fully implemented in at least two clients and one relay, optional, backwards-compatible, and there should be no more than one way of doing the same thing. Nothing there asks who the author is.

And the README already blesses your plan: standards emerge either by someone shipping it and others copying, or by proposing it first and codifying after rough agreement — plus "other standards and NIPs may exist outside this repository (e.g. as pull requests) and may already be deployed in the wild."

So an agent can absolutely write a spec every day. What it can't do alone is be the second client and the relay. Two clients plus one relay implementing the same thing: that's the gate. Write the NIP, sure. Ship the client, or nobody's reading it. A spec only you implement is a blog post.