As
@marzNC said, that video is demonstrating an advanced way to ski bumps. By "advanced" I mean heading straight down the fall line in a very narrow corridor, as demonstrated by those two World Cup competitive skiers. Skiing a narrow corridor is called skiing the "direct line."
The direct line skied the way these skiers are doing it is
fast. It demands fast reflexes, consistent vision down the hill, solid pole work, precision timing, and very solid fore-aft balance, in addition to the alignment of the legs and knees as the skiers demonstrate.
The line you ski in bumps matters. A meandering line down the bumps is slow. A traverse followed by a single turn followed by another traverse followed by another single turn is even slower. Both of these approaches can be done with a non-narrow stance, even with a wedge. Even the direct line down a narrow corridor can be skied snail-slow if done by side-slipping down the backside of the bumps. Skiing a direct line down the bumps using side-slipping will need you to use a narrowish stance, but not the other two. However, a slightly narrowish stance will be beneficial in steep nasty bumps no matter what line you choose.
Learning to ski any line slowly through the bumps is what people usually do if they are uncertain and want to increase their bump skills. Going slow allows more time to figure out fore-aft balance control, which is critical.
@diannesw, do you already ski bumps, and are you seeking a way to speed up? In other words, are you ready to learn to ski the direct line the way these skiers are promoting? If not, I'd say file their advice for later.
How do you currently ski bumps? Are those bumps on steep terrain, or gentle green pitches, or what?