Oh, it probably just needs to import the site service manually.
import { apiInitializer } from "discourse/lib/api";
export default apiInitializer("1.8.0", (api) => {
const site = api.container.lookup("service:site");
api.renderInOutlet(
"composer-after-save-or-cancel",
<template>
{{#if site.mobileView}}
Mobile!
{{else}}
Normal!
{{/if}}
</template>
);
});