The horizontal position generates drag as you slowly rise up or down, so it helps inhibit excessive speeds.
A good test to confirm that your are ascending in control
(But are still positive) is to exhale forcibly and more completely than normal. If a full exhale completely stops your ascent, then you are in control. If you are still floating up slightly, then inhale and dump a little from the bc.
If you inhale fully and your ascent stops, then you are a tiny bit heavy and you can drop your feet a little and give a kick and and move up a foot or two. I prefer this, than to add air to the bc.
I didn’t watch the video, maybe that is what they say. The real goal is to maintain your buoyancy imbalance to a magnitude that is less than the shift from normal breathing. If the imbalance is maintained within that envelope then you are never out of control, because manipulation of lung volume is sufficient to immediately arrest the change in depth.
For the last 10 feet of ascent, it may make sense to get vertical and stay negative and kick slightly so you can better see the surface and can be more aware of potential obstacles.
Interesting thought on controlling ascent that leads me to ta couple of questions.
1. Why not ascend face up and horizontal? It may be disconcerting at first, but allows you to see where you're going.
2. In thinking of failure modes, starting vertical allows you to flare to horizontal if the ascent loses control. Starting horizontal removes that option.