The argument is based on NF referring in the first person to doing things that JA did (debate in Iceland, conference in Norway), that other members of Wikileaks didn't do.
To me the fact that they wrote "other persons known" indicates that they are not sure if there were other members present.
It seems I wasn't clear before but this is exactly what I intended to imply: If they cannot attest that JA was the only member present at that conference then that line of argument is insufficient to make the claim they are making.