I was thinking exactly the same : I also don't get it (even though I totally get that someone may lose motivation to work on a project, and certainly has no obligation to continue. But this justification sounds a bit weird to me).

Could this mean that he has been approached by some "men in black" asking to insert some backdoor in the code or to stop working on it, together with a gag order ? (actually I also wondered the same a long time ago with Truecrypt, even though to my knowledge no backdoor has ever been found in further audits...)