The indestructible diary was meant by Eliezer to be a red herring for the duration of a couple paragraphs, until the readers were told it was the diary of Roger Bacon, not Tom Riddle.

Even that's strange though. I didn't seriously entertain the thought that it was Tom Riddle's diary or a horcrux. And then it being Roger Bacon's attempts to apply science to magic was super-awesome - it was sort of a Chekhov's Gun and I kept expecting Harry to gain some sort of insight or initial boost or secret knowledge from reading it.