I think RG was simply trying to say that the types or divisions between bikes is becoming smaller and smaller, hence the reference to "how thin can you slice the baloney". There are increasingly more types or style of bikes between road and mountain. A recent, yet not so recent, group is referred to as gravel grinders. Bruce19 mentioned cyclocross, another thin slice. The best advice is to try a bunch of bikes, borrowed from friends, rented, or test rides at the local bike shop (LBS).

Good luck - enjoy the riding!