Under Publish on a post, the accounts you post to yourself are explained in a note above the question of when the post goes out. It says that X or Instagram does not allow Magpie to post automatically, and that at the time you choose Magpie will email you, or everyone who can publish, a link to post it with the text and images ready. The spacing between the account rows, the note and the question is also more even.
Clearer note on posting to X and Instagram yourself
Under Publish, a note now explains that X and Instagram do not allow automatic posting and that Magpie will email you a link to post.