Why can't the square be flat in another way? Does it have to be like the way Prof. Loh have said?

What if the top is at another place like in this picture?

@victorioussheep Here's the thing: there are a ton of different cube nets out there.
You could fold all of these up and get a cube! And you're right, we can pick any side we want to be the top of the cube. In the problem however, because we can pick any side to be the top, why don't we pick the middle square? It makes all of our calculations a lot easier.Imagine if we were to actually pick the side you have designated to be the top of the cube. When you connect the two corners, you get a straight line! This is actually the exact line that Prof. Loh has in the video. It's the blue one. Do you see how that would actually be longer?
To be honest, you could pick any of the 6 sides to be the top one. But the middle square gives you the shortest distance between the two corners. It also makes sense that it would be the shortest, since it's a diagonal. Cutting corners is always faster than a straight path!

@quacker88 Oh, right, but in that question, I was just making the top the way shown in my picture, but I thought that it is the only way to do it.......

@victorioussheep Yeah there are a ton of ways to unfold a cube! You can try cutting out a cube net with paper and experiment with it on your own if you want