ZacD explained it pretty much on point.
You won’t really encounter these kind of issues described in the video unless you do something as extreme as smoothing a hard angle cube. These days polygon count is kind of a non-issue in modern game engines and while it’s true more polygons will take mroe performance, there are many other things that are much bigger performance hogs so unless you go crazy it shouldn’t be an issue. Therefore you should just bevel the cube instead of doing something as extreme as smoothing 90 degree angles like this, which circumvents any issues like that. But it all depends on case by case, no rules without exceptions and all that. That said, I have never encountered any issue he describes in the video while working in UE3/4 for years and only cutting where necessary to make a nice, flat UV shell.