Glenn said he also tried to publish it somewhere else and they forbid him from doing so, even though his contract says he has the right to publish stuff elsewhere if The Intercept doesn't want it.
They said "Our intention in sending the memo was for you to revise the story for publication. [...]
It would be unfortunate and detrimental to The Intercept for this story to be published elsewhere."
That's not forbidding him from publishing it, it's expressing regret that he appears to be unwilling to address the editor's comments on his draft.
Interesting, I didn't know that. But "detrimental to The Intercept" seems a little foreboding. It could be interpreted as them saying he's doing his job wrong if it's published elsewhere, and if he does his job wrong, he might assume there would be consequences.
That's not forbidding him from publishing it, it's expressing regret that he appears to be unwilling to address the editor's comments on his draft.