These kind of "1 hand vs. 2 hand" split proficiency weapons are kinda weirdly worded, but essentially it's not meant to mean that you always need exotic weapon proficiency no matter what, but that while it is an exotic weapon, you can use it as a martial weapon when two handing it. Also when it comes to layering effects the "top" effect overwrites everything else. So normally a katana is an exotic weapon that requires proficiency, but you are then adding on a magic item on top of it that completely overwrites it's properties to make it easier to wield without special training. To think of it another way, which do you think is stronger and takes priority, a mundane sword made of metal or just straight up magic?