I think what you describe is what people usually call accidental complexity. The complexity described in the article is about the incidental complexity, meaning the actual complexity of the features, rather than the complexity of their implementation.
Let's take an example, if hacker news added a new feature that allowed people to delete their account, then you have to decide what to do:
- do you delete the user's comments?
- if yes, what do you do about people replying to that comment?
- do you delete the user's submissions?
- if yes, what do you do anout people commenting on the thread?
- do you downvote the submission / comments that the user upvoted?
- do you unflag the submission / comments that the user flagged?
- do you keep a copy of their yc applications?
- do you keep their email address along with their karma, so that if they open a new account with the same address, their karma will be restored?
As you can see, adding a single feature means that you have to figure out how it is going to play with all the other features. That's what the author means when he says: that makes the cost of implementing the N+1 feature closer to N than 1.
Let's take an example, if hacker news added a new feature that allowed people to delete their account, then you have to decide what to do:
- do you delete the user's comments?
- if yes, what do you do about people replying to that comment?
- do you delete the user's submissions?
- if yes, what do you do anout people commenting on the thread?
- do you downvote the submission / comments that the user upvoted?
- do you unflag the submission / comments that the user flagged?
- do you keep a copy of their yc applications?
- do you keep their email address along with their karma, so that if they open a new account with the same address, their karma will be restored?
As you can see, adding a single feature means that you have to figure out how it is going to play with all the other features. That's what the author means when he says: that makes the cost of implementing the N+1 feature closer to N than 1.