This is not accurate. Any time you have multiple people sharing a physical link you end up with a marginal cost for bandwidth in any sensible economic arrangement. If you insist on having every part of your connection dedicated to yourself, you're effectively back to a circuit-switched network with the corresponding orders-of-magnitude price hike in providing it.

For example, in the UK the last mile is usually supplied by the incumbent (BT), who also (virtually) route that traffic over a private IP-based backbone to ISPs to provide the backhaul. They charge ISPs by capacity. So these ISPs effectively have marginal costs that vary throughout the day according to demand, since they have to pay directly for an appropriate (fixed) amount of capacity to be available.

We also have LLU (local loop unbundled) ISPs who still use BT's last mile but provide their own equipment directly at the exchanges, thus effectively providing backhaul straight from the exchanges. They may only have a fixed cost for bandwidth depending on how they provision those links. But if they are buying capacity from anyone else, they will be charged by how much they are buying, and suddenly they have a marginal cost again.

The article says that Netflix are complaining about the situation where Netflix have brought the traffic to the last mile. In this specific situation it makes sense. But in the general case, ISPs still have marginal costs in providing bandwidth.