I think it comes down mostly to spreading the word that this community exists. For a lot of people, Reddit is the "default platform" for topic-based forums. Changing that mindset is difficult.

Personally I wouldn't expect forum communities outside Reddit and the official Rust forums to really grow much unless Reddit dies. Maybe there's a way instead to somehow get the official forums connected to Lemmy? That way you'd get all the users of the forums right away without needing to ask those users to change platforms.

Edit: what I think would be the dream solution is the official forums looking and behaving the same, but being accessible through ActivityPub-based services like Lemmy. It would only really let forums users post specifically on the Rust forums and browse Rust content on the site, but would allow federated users to also communicate there through Jerboa/Sync/etc using accounts made on other federated servers.