The author "incremental reading" technique directly contradicts this. I don't know who's right though.
Here is a YT video of him watching a couple seconds of dozens of YT videos to extract knowledge : https://www.youtube.com/watch?v=zOJyO7QDEX4
I'll admit it seems absolutely counter intuitive!
When it comes to SuperMemo though, you're not managing learning. You more put in what you want to learn, push a lever or two (priority/intervals) and supermemo takes care of the rest. SuperMemo is way less stressful than normal learning because when you use it and are doing reps, you can focus as much as you want on what's in front of you and move on when you want to. The costs of switching between things are near 0 since you'll always see the thing again (unless you decide not to).
For working on more than 1 project at once, I think that's pushing it though with incremental writing (https://supermemo.guru/wiki/Incremental_writing) it becomes more feasible to work on multiple things in parallel incrementally (also enjoyably).