Primary topic is for two different types of topics that both have the same name. It would be one thing if there was Persian Empire and Persian Empire (book). This is not the case. The primary topic of "Persian Empire" is a Persian imperial state. There were multiple ones. Each have their own page. Therefore, this page is devoted to the -name- and its -use-. You cannot say that it can only talk about one use and therefore be a redirect. That is not acceptable in policy nor is there any precedence for such an action. See WP:WEIGHT for how to deal with the amount of emphasis put for amount of uses, which is the only thing even close to what you are talking about, and it would not allow for a redirect as that would be a clear violation of WP:WEIGHT.