It says in the article that Fliescher and Libby only had lunch together once and the matter came up over that lunch. Given that Fliescher certainly has a schedule that he can refer to to give him the date of his lunch with Libby, it is not too hard to believe that he can say with confidence which day he heard this from Libby.
Only if the lunch was the only time they ever spoke to each other. I'm not buying it; they worked in the same place and I just don't think most people are guaranteed to remember the surroundings of this sort of conversation without any chance of error. We also have to consider that Fleischer was anxious to come up with something definitive to appease Fitzy, who did not want to give him immunity.
No, it does not say that. Read it again, please. It says it's the first time they had lunch together, not the only time.