I don't know who Rosti's sources are, but since he is not an employee of Nintendo, presumably he has them. I don't know if he has ever provided leaks before, I only came to GAF in 2013 and don't recall if he's provided leaks since then, but when he says his health is adversely affected by a fake leak and indicates it's related to information he has from his sources, I believe him, however naïve that may be of me. (I genuinely mean that, I can absolutely be naïve at times so perhaps I'm being unreasonably trusting here also.)
Since I believe him that he had information and sources then, I then believe he has good information now when he's making positive statements about the system. As I said choosing to believe him or any other insider is going to be a down to the person's decision of what to believe, so my choice to take stock in his favorable words about NX is simply that, a personal choice. I hope that makes sense.
FWIW, even if I am saying that he didn't specifically make any claim the leaked controller was real, I'm only meaning to politely disagree and not to disparage anyone's opinion of whether his information is valid or not based on the evidence at hand. People are entitled entirely to their decision of whether or not he's a valid source, I just wished to point out the basis of the initial accusation (Rosti insinuating the leak was real) as not necessarily accurate.
I don't think we can know that without knowing the extent of what he knew at that time and whether he was being accused or something of leaking information, or what the reason was, but, I have no reason to doubt he was telling the truth about being impacted by it in that way.