My comment was speculation as to how Evanstonians viewed NU and it's football team, not its high school team. They appear to be supporting ETHS just fine. NU not so much.
Most of these people are here to experience the stadium at a great price as the others have said. According to the press release Ryan put out, it's near 27k seats sold and will probably be the highest attended high school game in Illinois history.
I wouldn't be surprised if we see this happen on an annual basis; eventually some of the people that experience NRF for other events (other games or seasonal events or concerts) may decide to buy NU football tickets.
That's the whole point of making NRF a true "40-60 days a year" facility instead of just a "7 days a year" facility. There will be a lot more community events at NRF and eventually for tens of thousands of people, going there will be a lot more of a regular thing than it has ever been. And some of those people will eventually show up for NU football games after going for other events (that's the theory at least).
The point is to bring in as many people over time and show them that NRF is basically one of the best stadia in the country for experiencing anything (almost certainly will be the best college football stadium experience for decades to come).