The linked article isn’t as bad as many but it still doesn’t make plain the real issue here.
The bottom line to this story is that only one of the released documents was damning of the Heartland Institute and that document, the “strategy memo”, was a transparent fake.
Gleick did not just lie to Heartland in order to steal their documents. He lied and stole genuine documents and then bundled them together with a forged memo in order to more convincingly pass off the forged memo as real.
The leftists are claiming that Gleick did nothing more than steal and distribute genuine documents that prove Heartland to be a bad actor.
Saunders, in the posted article, fails to directly rebut this lie.
... and any article that falls short of your summary - is giving cover to the lie.
As this article does. Semi-truths to hide the bald faced lie.