I tried converting some ePubs to mobi via calibre and set the Left and Right margins to zero, but it had no effect. You can adjust the margins by changing the Words per Line setting between Fewest, Fewer, and Default. Default is the setting you saw in her video and equates to appx. 1/4 inch L&R margins. Fewer is appx. 1/2 inch L&R margin, and Fewest is appx. 3/4 inch L&R margins. These settings are ridiculous of course. Amazon should have set Fewest to no more than 1/4 inch L&R margins and the other two to even smaller margins.

I'm going to explore mobi a bit better and see in I can find a way to decrease the margins in converted books.