I had a JW relative investigate by speaking to one of the Elders™ who came to the house. He said that he and other congregation members had observed Mr Scully smoking, and so asked the question "Do you consider yourself to be one of Jehovah's Witnesses?" - when Mr Scully said "no", that was taken as a verbal Disassociation™. I still wanted to know about the claim regarding writing a letter - and apparently the rules changed around that time and they started making DA announcements for verbal as well as written DA statements. They also started DAing people "by their actions" - so say a JW joined the army - they would be automatically DAd "by their actions" because their actions were in direct violation of WTS standards of conduct. Apparently the DA was on both counts - actions and verbal statement. Whoever started the rumour about the letter was under the impression that DA statements were only valid in writing.
I think there will be lots more people DAd "by their actions" - it saves the Elders™ from having to investigate allegations of wrongdoing, you only need two Elders™ to allege that a person made a statement to the effect that they do not consider themselves to be a JW, and it prevents any messy legal proceedings (slander/libel) if a person is labelled as Disfellowshipped™. This is one reason why the announcement does not identify a person as DFd or DAd any more - only that they are "no longer one of Jehovah's Witnesses" because there's no overt defamation of character taking place. The whole concept of "DA by one's actions" makes any action that violates WTS rules a completely voluntary decision to disassociate on the part of the individual.