I guess I was expecting the code shown here to use ? to simplify the error handling? Or was the idea that instructor or student did that.

Certainly, the first match seems suitable for using ?:

    let mut username_file = username_file_result?;

The second one, because of the compatible types point, is slightly less straightforward. I changed it to:

    let _ = username_file.read_to_string(&mut username)?;
    Ok(username)

which seems to compile and run OK. (I presume it's OK because any error would have been propagated before the return value if it had occurred).

Not sure whether you intended to do the simplification, but it reads as if you did, either by showing both or just the simplified version.