Yeah, this exercise isn't about building a sphere, it's just about learning to get information from other objects that are close to each other. Reread the notes of the exercise and the video if this isn't clear. But basically you can use the corners that are really close between boxes as they will have almost the same perspective, so copying the perspective of the corner you have next to the box you're creating will give you a correct line. This eliminates guesswork, as you can just copy the line without worrying about making perspective mistakes.

That's why one of the mistakes on this exercise is to not put the corners between boxes close, because if you don't put them close, you can't take advantage of it.

Thinking this exercise's objective is to make a sphere is a common misconception. Don't be misguided by the exercise example, look closely at the instructions you're given.

I've seen misinformation on reddit like this exact thing several times, careful with the information you're given there, don't just accept everything you read as truth. If you have doubts, ask where that information came from, and try to understand it.