Without further work, a template in the Startup Folder is loaded as a global template. Yours is "loaded." It takes macros in the template to make it become the default basis for new documents. When a template is loaded as an Add-In, it becomes a secondary global template. It is not the Attached template for the open document absent special code. One example of such code is in the sample Add-Ins, another example can be found at A Global StyleSheet in Microsoft Word?

Under the Developer Tab, you can see the button for Document Template. That opens the legacy Templates and Add-Ins dialog. In that dialog, you can see which template is the attached template for the active document as well as any loaded template Add-Ins (global templates). That dialog is the easiest way to see if a template has been loaded on your system.

Attaching a different template does not change any document content directly. The way to get layout and text from a template into a document is generally to create a new document based on the template, not to attach it to an existing document.

The sample Add-Ins are designed to create new documents based on a specified template other than the normal template.