That someone knows a name for something that they're looking for is an idiotic reason to dismiss the idea that it could be in a largely-unexplored region - just look at the legend of El Dorado or the Fountain of Youth. That Obi-Wan has a location where he thinks the planet is supposed to be would be a better reason to dismiss the idea that it's in the Unknown Regions, if the spot that he thinks it should be is not in them, but it's still a very iffy reason to do so since he could have the location wrong, and on top of that the area he points to in the film at least seems to be in or near the area that many of the published galaxy maps assign to the Unknown Regions.