No, you absolutely do not have access to "all publications" - for example, you cannot ask the Library of Congress to send you their copy of the Magna Carta. Some things are too bulky, expensive, or fragile to ship. Remember though that you asked about some academic's "magnum opus", which is unlikely to be restricted that way.
No, not all public and private libraries are members of the OCLC. A search of http://www.oclc.org/contacts/libraries.en.html gives 15K OCLC members and 63K participating libraries around the world. The library at the National Museum of Mysteries (see http://web.randi.org/swift/paranormal-themed-festivals-bring... ) is not listed as one of them.
Remember though that your scenario concerned a state university, and those definitely are members of ILL.
"Technically one could argue that I have "access" to those books."
Yes. Though 'technically' you have access even if you have to fly to the Vatican library to see something in person. I would add the qualifier "relatively low-cost". It's costs me about $12 to get a copy of a journal article though ILL when it costs me about $35 to get it directly from the publisher. The main difference is that I need to wait a week or two. Books, oddly enough, are free.
To get back to why I joined this thread, do you accept that your scenario of having to visit a state university library in person to read someone's "magnum opus" in book form is unlikely enough that it is more fanciful than meaningful?
As you are also not an academic, I instead offer a better scenario. Many Masters and PhD dissertations are deposited in the univeristy library but otherwise unpublished, and even un-indexed. For some of those you may have to visit the library in person to see a copy or even learn that it exists.