A few months ago I did something somewhat similar of what they show in the article, but with Photoshop.
From Shotcut I exported as a jpeg sequence the part where the man walks in front of the screen, then imported each jpeg (30 images total) in photoshop to erase the left part of the background. Finally imported my modified images in Shotcut and did this transition.
I just downloaded Krita to test it. They say in the article you can use keyframes. That sounds interesting. It might speed up the process.
But damn… Another program to learn! 