A file handle is already an abstraction. Sure, it's usually attached to physical bits on a magnetic disk, but Unix is explicitly designed so that a "file handle" could also be many other things. It could be a pipe to another program. It could be a network socket.

The professor in question seems to have a problem making arguments. A file handle is in no way tied to magnetic-based storage, and hasn't been for 30+ years. If you want to say that a file handle is a poor abstraction, you must at least show a better one. I can see how "it's just a big string" could be handy in some places, but it would be no good in others.

For instance, on a networking socket, the "big string" would be constantly changing as data gets passed back and forth. You couldn't treat it as "big string" because a typical string isn't affected by external systems in typical operation. In other words, you can't completely abstract the underlieing operation. OTOH, the only difference (from my program's point of view) between a network handle and a file handle is how it's opened. After that, it's all the same operations.