That's the odd part - it reads like a DOJ statement plopped down at the end of his summary.
Yes, I thought that was a bit of an odd statement by him.
Statements about what a reasonable prosecutor might do should probably be left to a prosecutor, not an FBI official. Especially because prosecutorial discretion is complicated, and reasonable people will disagree on how it should be applied. Look at the controversy surrounding Obama's immigration policy. Even peaceful, hardworking, non-rapey illegal immigrants have violated the law and could legally be deported. Should we deport all of them that we can find? College kids smoke pot in their dorms. If they get caught, should we always press charges? If we don't, some will argue that prosecutors are rewriting laws rather than enforcing them as they were enacted by the legislature. If we do, some will argue that we're subverting the spirit of legal pragmatism by overreacting to ticky-tack offenses.
In this case, there's a statute that makes gross negligence a violation. According to Comey, that statute has generally been treated a bit like criminal laws against pot-smoking (in states that treat that leniently) -- charges aren't brought without the presence of other factors, and those other factors weren't present in this case.
But should a prosecutor nonetheless bring charges in a case with a profile this high? The FBI can give its reasons for recommending that no charges be brought. But to say that no reasonable prosecutor would disagree with that recommendation seems like an overstep, IMO.