You make it sound like NBC is some sort of obscure specialist service, but it turns out that they're actually a mainstream national broadcaster, available not just over the air but also on just about every satellite, cable, and streaming provider in the country: https://en.wikipedia.org/wiki/NBC

Not only that, but they're the official Olympic broadcaster in the USA! Around 22m watched it on their broadcast services, and a further 3m on streaming.

In reality, an overwhelming majority of Americans watching the opening ceremony were doing so via NBC.