this is exactly why I said that asking about the security of GitHub is not interesting, outside of knowing whether the servcie itself is secure (and it is, to the best of my knowledge).
Making the decision to outsource something to a 3rd party is part of the risk analysis of the operation (it includes all kinds of elements, legal, IP, privacy, subcontractors to the 3rd party, ..). Thi swill not be done by information security who can only speak of information security.