Clarification by misnegation: The view from pragmatics

In a comment on Mark Liberman's post "Clarification by misnegation", Stephen Hart makes a point that the rest of us have missed (or at least haven't raised), and that deserves wider attention:

I may be missing something here.

Slightly restated, Trump said, originally: US Intelligence says it is Russia. Putin says it isn't Russia. I don't see any reason why it would be Russia. (What would Russia have to gain?)

The new statement seems to be:

US Intelligence says it is Russia. Putin says it isn't Russia. I don't see any reason why it wouldn't be Russia. (Everybody does it.)