The closest I could find so far is the Knife Flat Smudge from TheFlow collection, though I wonder if there’s anything even closer to that flat bristle brush in the video.
P.S. In case the URL doesn’t take you to the right video, is the third one called First Sketch.
I don’t have an account so can’t open that video but I have an idea. Open the Brush Selector (F6), make sure it’s set to “all” at the top and type the word bristle in the search box at the bottom. Krita comes with many brushes, including bristles. You may already have what you’re looking for.
I started using Krita only in the past few days, I’m already loving it. I’m only scratching the surface, but it seems already a beautiful software with an equally lovely community.