I'm not sure if this would work, unless it was done in secrecy. From what I've come to understand, rhino horn usage has a superstitious component to it. If it's known a sample doesn't come from a rhino people wouldn't be interested.

In addition, poachers know how to make people stop doing things: keep killing rhinos.

The point is to flood the market with fake horns and make them indistinguishable from real ones, while at the same time broadcasting the fact that you're doing so. This will spread fear, uncertainty, and doubt about the genuineness of any product that claims to be rhino horn.

This is classic flood-the-market-with-cheap-alternatives-to-destroy-the-competition business strategy, except the goal is to destroy the entire rhino horn market.

Is it actually feasible to 3d print a realistic rhino horn from synthesized keratin? Googling keratin 3d printing mostly turns up Pembient itself. One would suspect that keratin printing would have plenty of more humble applications (synthetic hair, nails, biomimetic prosthetics).

Synhtetic rhino-horn is a clever idea, but one could suspect that once it goes "mainstream", the demand will also rise. Or am I completely wrong?

That is touched on in the final paragraph