Initially I thought that the secrecy was only to shield information from flowing outside of the company but if you dig deeper there is a much more useful side-affect. If you are part of a 4 member team that is working in a 10,000+ employee organization, it is very easy to get lost in the bureaucracy, processes and fooling yourself that being busy is the same as being productive. However, by making your work secret; it effectively limits what you can talk about with other people within the organization and limits other people from butting into it. This allows the 4-member team to focus on their work, talk to one key person at the top to get through large-organization bureaucracies and be as effective as a 4-person startup.