The screenshot literally says, “Read the rest of this story with a free account. Already have an account? <sign in>” and prompts you to link your google or Facebook account.
The Wikipedia dark patterns linked describes them as, “A dark pattern (also known as an anti-pattern or ‘Deceptive Design’) is ‘a user interface that has been carefully crafted to trick users into doing things, such as buying overpriced insurance with their purchase or signing up for recurring bills.’”
I don’t want to defend Medium, but their approach hardly strikes me as deceptive or particularly tricky. Annoying? Yes. Dark pattern? No.