As text is made up of individual letters, Design Space sees those letters as individual objects when it comes to slicing. And you can only slice two objects as a time. The video goes through the process of welding the text to make it one object.
You would use the method to slice into more than two layers. (For example if your image had several shadows/offsets.)
Just remember that once you weld text in Design Space it converts it to an image. And that means you'll no long be able to edit it. So be sure you have your text exactly how you want it before welding. Once you close your Design Space project you will no longer be able to use the Undo feature to unweld the text.