That ‘speed’ design is using a lot of poor practices. I wouldn’t use it as a basis for understanding anything.
Neat for inspo, but nothing else.
The issue is how messy their workflow is.
In theory, what they’re doing is ok, but they aren’t conforming to any reasonable grid.
rule #1 of level design: obey the grid.
I think they get away with it because it’s a medieval setting, so by offsetting their assets a bit you can stop them from perfectly overlaping, but as @darthviper107 said, that will become a problem when you are further away, and you can see them in the video spending too long just aligning things together.
I’d suggest having a look at this GDC post by the environment and level designers on Skyrim. By having modular assets that conform to the grid, you can actually get better results than that speed design video, without having to mess about and making sure everything is aligned manually. It also means you won’t run in to z-fighting as you’d’ve put your assets together properly