I think its kind of worse than tanking it. The way its explained, if she didn't dodge it she would have literally fucking died from that attack. They foreshadowed this sort of thing a while ago where that one dude told she she was basically putting her life on the line in every fight because of her retarded fighting style. Isn't it great that Vivio almost made her own mother murder her in front of thousands of people?
I have to a agree, the explanation is kind of nonsense. I don't even know anymore if the damage is still supposed to be simulated in these battles since in the past we have gotten contradictory information from the story, so it isn't certain if she was really at risk.
That has been one of my problems with this series from early on, with the ridiculous power levels of the characters making it really hard to fit this in the context of earlier Nanoha stories. That and making it a random martial arts tournament out of nowhere never really sat right with me. One wonders if such a tournament is the right place to risk your life in a silly exhibition match against your mom, but hey.
The cast has also ballooned to even more ridiculous numbers than Strikers, which already had problems keeping a single good story going, instead opting to juggle several with less than great results. All these super powered little girls start to look the same over time, not to mention they retroactively make the original cast feel less special, but that goes back to how hard it is to pin down how powerful they are really supposed to be.
In short, this series is kind of a mess isn't it.