Remember, while we often say that ranges overlap, population density was also considerably smaller than today and physical barriers to travel were of a greater magnitude.
If you have ever hunted, you will understand that even when you are in an area where it is known that your target species is available, it is often the case that you will never see a single critter. Even in our modern manicured wilderness, it's not unusual to find tracks crossing your path of a critter you never saw.
With low density, it's possible that two species could live their lives in relatively overlapping territories yet not be aware of the other.
Aslan is not a Tame Lion