I'm not sure, since I have no idea of the rationale behind the rule. Might be because Apple wants apps to seem like they just "always work", rather than requiring them to be fixed for each release? In any case, I've heard people get around this rule with clever wording, as long as they don't explicitly mention the new hardware/software. For example, "dark mode support", "full screen display", etc.
lloeki elsewhere in this thread mentioned that Apple is approving some apps that mention Mojave in their release notes. Whatever the rule is, Apple isn't applying it to everyone.