Its out of context because Plagueis was not injured at that time.
Actually I see a connection between that revelation and the events that took place during Plagueis's battle with assassins. Plaguies's powers didn't diminish by the wounds inflicted upon him by the assassins and this is why he was able to "all but atomize" them. No rocket science here.
I am not going to end up embarrassed because I can connect the dots better then you.
When injured Plagueis was forced to divert his attention to keeping himself alive which would have demanded both physical attention and Force exertion - meaning his ability to attack with the Force would have been diminished.
What the novel is saying here is that those injuries were not permanent, Plagueis recovered from his wounds and became more powerful than ever, refuting Sidious' assumption that it may have permanently weakened him.
All in all you have failed to understand (or deliberately overlooked) the difference between temporary and permanent effects, whether that is a cause for embarrassment is up to you to decide I suppose. Its certainly not a surprise.