Hi Chloe!
Remember to focus on the conclusion of the argument. The conclusion here is that Robert's debate performance was as good as Britta's (when you take reasonableness into consideration). So we're looking for an answer choice that is necessary to the conclusion that Robert's debate performance was as good as Britta's. Answer choice (E) has nothing to do with that conclusion and so is not necessary for it. Answer choice (E) may be necessary for Britta to win the debate, but it is not necessary for the argument, which is that Robert's debate performance was as good as Britta's. Always focus on that conclusion!
"Regardless of their ability to bring the facts to bear in those arguments" just refers to the author's contention that "reasonableness" should be a separate consideration from "command of facts." Both can contribute to how their debate performance is assessed, even if they are considered as separate skills.
Hope this helps!
Best,
Kelsey