Arvedui does indeed get about two dozen mentions - one reason why he gets more than ultimately more important people is that his story is sort of told twice in Appendix A, once in the history of Arnor part, once in the history of Gondor part. In LotR itself, the Index claims he's mentioned exactly once, and in a footnote at that.

And yeah, if it does include the Appendices, it would explain why Arvedui is in the list and Elbereth, Beren, Luthien, Earendil, and so on are not, even if they're mentioned many more times in the text - Elbereth is mentioned in at least ten different places according to the Index. I would suspect Helm Hammerhand makes it to 29 mentions without counting Helm's Deep - you'd get to a fair few more then. Helm gets to 21 mentions in four pages in the Appendices, so then 29 total isn't that much. People like Eorl, Thorin and Durin likely get most of their mentions in the Appendices, too.