That's only citing evidence that's been public since the 40's (which was inconclusive, but suggestive). The author apparently didn't hear about the recently declassified stuff, which says the offer came through diplomatic channels (which typically happens before public, formal surrenders), and the White House knew, but ignored the message.

The thing you linked cites a source that had access to the actual message and published it in August 1945. Then it tries to debunk its now-verified authenticity.