Anything is a shape in Visio, including pictures and text.
When working with Visio, you will probably want to add text, photos, or clip art.
Although these are not shapes created by Visio, in Visio all of these things are shapes nonetheless. If you type text on an empty part of the page, that text will be a 2-D shape, and it will have eight selection handles when selected. If you select an imported picture, it will have those handles too.
Let's face it: anything on the page is a shape to Visio. Anything.