Anyone who knows anything about photography understands that the sensor is 20 MP.
The way that a 16:9 aspect ratio is achieved is by chopping off the top and bottom of the full 20 MP image
It shouldn't be a surprise that you get less megapixels when some have been cropped from top and bottom of the image.

You can crop to any aspect ratio in your computer.
Crop high or low, do what you like.
There's no need to give up some options when shooting.

True enough @Labroides, I don't think that Youtuber had experienced pilots as his target audience.  But kudo's to him for creating some content to share with the drone community.

Nice video!