Could the government direct librarians to purchase only Bibles, with explicit bans on purchasing the Quran?

I assumed by “First Amendment” you were talking about the Free Speech clause, since that’s what the article is about.

Government speech of course remains subject to other constitutional provisions. But the standards for these provisions are quite different. Your hypothetical raises the possibility of an Establishment Clause violation. But to prove that, it’s not enough to show that the government is discriminating as to view point. It has to be enough to amount to an establishment of religion.

And of course the Free Speech Clause covers basically any subject, while other constitutional provisions are much narrower. For example, a government-run book selling platform probably couldn’t exclude people from trading books on Brutalist architecture. But a school library could have such a policy.