Your trying too hard to compare speed ball to sprinting. STOP IT NOW 
It’s just a way to build a base of fitness. That’s all nothing more. Think of the body wieght exercises as circut training. After I get to the 3 or 4th round of this brutally simple circut my arse is on the wall and I am panting and sweating… It’s not a means to an end it’s just another way to get fit and that is pretty important.
Everyone needs fitness… sprinters gymnast basball players and the list goes on… We’re athletes and we need to be fit as well as strong and powerful.
So the point is that there are a million ways to get fit. This is just one more way to do it. If it works for you and it isnt hurting anything, then why the hell not?
As far as the time was concerned I think Jim Bradley just found after trial and error that is what worked best. I read his book and he tried different times even going for 30 straight!!! and this was the time he found worked best…
I dont use the speed ball right now but it’s fun and it gets the heart rate up…