Well that is because she would be using magic to enhance her power, that is why i mentioned that she could have been a descentant of magic users, what if they went from being spell casters to being assassins by channeling their magical power into their weapons and thus her stronger attacks are fueled by magic. do deal magic damage means to have some level of magic power in you but only a few have the raw power to harness it on hand like a real mage could do
some being use magic like a mage while others like to channel it through their body to enhance it or their techniques. Kat seems to be the later as she gets physical, but her damage is magical in nature, a direct contratition to her adoptive brother, talon who I doubt have any kind of magical power what so ever.
But speaking in regards to Talon, he could be using Ki or runic energy to hold his blades up during his ult. Why do I say this? Well because Riven, another lowborn noxian can use "Ki Burst" and that deals physical damage in spite if it being a non-solid attack, Along with with holding up her shattered blade and using wind slash ult, without dealing any magic damage what so ever. with is the opposite of Kat on some level.
Not everone on Runeterra need magic to pull off some crazy feats.