Earlier quoted context omitted.
I'd say the Substack UI is messed up if (a) there's a hidden input box that automatically changes the selection, even though the user cannot manually enter information there, and (b) there's no confirmation screen to confirm everything is correct. It shouldn't matter that a password manager exposed the problem.
The use of the word "hidden" in the article is misleading. The field is visible and editable by the user -- it is just styled in a way that does not make it clear it is an editable field. There is a GIF in the article showing the author changing this field value manually.