That doesn't apply here,
@Binbo : they were matched before that, or Trunks was even at a disadvantage. If he reduces his speed when they were the same, it will produce the same results as it did in the official timeline.
There is more times where the author forgot about things, like the androids shooting energy attacks even though they have no energy so they cannot do that, but that is not as bad as there is so many videogames where they can.