The large section is also just a low effort version. I'm sure this could have been inserted instead as a collapsed section, a comment, document metadata, .1 sized font, Alt text on a single pixel image, or any number of other ways to disguise the prompt.

It's an arms race for defenders, and even someone paying attention might not notice. Not to mention, you don't even have to open the document. If it's in a SharePoint library, or attached to an email, there's a good chance it's in scope for whatever random 'assistance' copilot will attempt automatically

Word's interface doesn't help. Default view gives you almost zero information on section breaks, and even if you turn on formatting marks they behave in an unintuitive manner and there's some edge cases where Word will still generate a new page after a continuous section break unless you set the font size after the break to 1.