Thanks for the thoughtful comment, Metin.
You are right, this approach is about workflow. But think about it this way: let’s say you have 10 hours to dedicate to a project, and you know that the final deliverable is going to require 6 hours of that time to be built, you’re left with 4 hours for any step that could enrich it before. Now, if you’re able to reduce those 6 hours to only 1, then, you just won 5 extra-hours to dedicate to the earlier steps. And guess that? This is when you’re designing the end-to-end experience. Hence the link between the efficiency of your workflow and your ability to focus on the final user experience.
As for adaptability of the UI kit, one important detail: this one is made for one product (BlaBlaCar, long-distance ridesharing). It’s not a neutral system that anyone could reuse, but a very specific set of components that been designed for this one product.
Hope that clarifies your questions.