When a Page object is not in context, you can use the global page function:
Hugo almost always passes a Page as the data context into the top level template (e.g., single.html). The one exception is the multihost sitemap template. This means that you can access the current page with the . variable in the template.
When you call the Summary method, Hugo renders the page content including shortcodes. In this case, within a shortcode, the global page function accesses the Page object of the section page, not the content page.
If Hugo renders the section page before a content page, the cached rendered shortcode will be incorrect. You cannot control the rendering sequence due to concurrency.