You're complaining about them pre-filling your email when you have an account? How is it a dark pattern? It's convenient. It's not like they're automatically signing you up to the publication.
What account? I do not have a Substack account. I might have subscribed to someone in the past who is a Substack customer. Does that give them the right to automatically fetch my email address for all of their other customers, too?
It's creepy.