Allan, I watched the video and tried to do the same thing to one of my images. But, it's not behaving the same as in yours. I open an image, select the perspective tool. Change it to Dual Pane and Source. Then place the handles on the points I want to distort. Then change back to Destination (like you did in your video) only mine distorts the whole image as it did before, leaving areas transparent. It doesn't behave the same as in your video, ie. just distorting what's within the constraints of the mesh tool.
I must have something set wrong, but it's very frustrating that I can't figure this out. Will likely abandon this for now. I was hoping to find a tutorial so that I could understand how this works, but my Affinity Photo is not functioning the same way as yours is, while I'm doing the exact same thing you're doing in the sample video.