The the above diagram is a Component diagram, which is used to "decompose each container further to identify the major structural building blocks and their interactions".
On this diagram only Mainframe Banking System Facade is documented and therefore has a page to which its diagram element links.
In the source diagram the "Mainframe Banking System" diagram element is linked to the "Mainframe Banking System" element on the System Context Diagram using this element link: data:element/id,id,Ht1M8jgEwFfnCIfOTk4-/mainframe-banking-system.
As such, both elements link to the Mainframe Banking System page in the generated site.
The "API Application" container is linked to the "API Application" node on the Container Diagram using this link: data:element/id,id,fufg5MHGldTBZNh1nXvk/api-application.
This results in the "Mainframe Banking System Facade" page being an immediate child of the API application page.
Without it there would be an intermediate "API Application" label representing the container.