I think this quote (regarding the California High-Speed Rail Initiative) is far more revealing than Matthew intends it to be:

[I]t seems to me that the sad reality of politics is that it would be irresponsible for advocates of any large-scale infrastructure project to do anything other than present unrealistically optimistic measures.

Translation into English: it would be irresponsible for advocates of at least some policies Matthew Yglesias prefers to tell the truth. Oxford’s New American Dictionary defines “responsibility to” as “a moral obligation to behave correctly toward or in respect of.” So Yglesias believes it would be immoral for advocates of some of Yglesias’ preferred policies to be honest. I’m glad we have that cleared up.