He's a politician, and his stated reason for the veto is not necessarily his real reason for the veto.
This suggests that politicians and donors sometimes collude to deliberately misrepresent their views to the public in order to secure election.
You're also missing the first half, "don't write if you can speak, don't speak if you can nod, and don't nod if you can wink." The point is not to commit to anything if you don't have to.