Because it is almost a certainty that were Netflix to manage to provide the fiber to send their data to their subs, it would be based on internet technologies and protocols. You know there is a small-i internet and large-I Internet, and you can have one that is limited in access while the other one is the worldwide interconnect of all the small-i versions, don't you? (And before you point out that ISP has a capital 'I', that's because it is an acronym, not necessarily because it is only talking about large-I internet services.)
The point is, anything that connects a million users together is not a "dedicated point-to-point link". It is more like an internet, and when one provides service over that internet, one is for all intents and purposes and ISP, or very much like one. Especially if one is doing all the last-mile connections and other companies want to get their data on your fiber. That is, after all, what people are trying to get cable companies to do -- open their pipes to other providers.
By your logic, a cable TV network (with no data services) is an ISP because they are running a backbone and providing content.
Yes, if a company is providing a service based on internet protocols and technology then they look very much like an ISP. You plug your internet connection into their hardware, access their servers, ditto.
Unfortunately for your argument here, cable TV networks are not distributing their standard video content using internet protocols, and one does not connect to a cable TV video server to get it. They use ATV standards to distribute their legacy products, which removes them from the ISP look-alike competition. Even for on-demand services where there may be an internet-based upstream connection to make the request for video, it is still delivered using ATV. At least that's how Comcast does it.