I am paywalled from the article, so maybe I’m misunderstanding the quote you pulled due to lack of context, but you also can’t just double the acceptance rate over night. Even if you could admit more students and cover their tuition, you run into a number of issues, e.g.:
1) Students have to live somewhere. More students living off campus will drive up apartment rent in the area. You can build dorms to increase the housing supply for students and reduce the demand for off campus apartments, but that requires large building projects.
2) Similarly, increasing the student population means more class rooms, teaching labs, and possibly larger lecture halls for the most popular classes (e.g., calculus courses taken by every STEM student). That means more building projects.
3) You need to hire more faculty. While you might be able to get away with adjuncts or postdocs teaching a few things, people that want to go to these schools want to have the opportunity to interact and learn from the top experts in their field, i.e. faculty professors. Expanding the faculty is not an easy thing to do overnight. Even if the will is there to grow, departments are highly selective when it comes to hiring. The person you are hiring is joining the university potentially for life. Moreover, if you choose wrong and deny them tenure, the result blows back on the department. Hot tenure track candidates get many offers, and they don’t want to risk moving somewhere and building a lab for 5-7 years only to be denied tenure and having to move their life elsewhere (they’ve already done that quite a bit throughout their student and postdoc days). Moreover, those candidates need to be courted with resources like spaces for their research lab, so, again, more construction is needed.
4) Where do you build all these buildings? Many of the Ivies are located in major cities (e.g., Boston, New York, Philadelphia). There isn’t a lot of open space to build big new buildings. Moreover, while they often own buildings adjacent to campus and lease them to businesses and tenants, decimating a city block to displace these people in favor of building more lectures or residential halls is a bad look politically and a hard sell to local governments. These schools already draw complaints from the locals about their expansion gentrifying areas and driving locals out through the resulting rise in rents and taxes. Even suburban Princeton has been rapidly consuming their green spaces lately to build new facilities. An option might be to start opening separate campuses, but that’s unappealing to their prospective students.