The person's point was, the item was made for physical damage dealers, both in spells and autoattacks. Katrina does next to none. So if it was indeed custom made for her, why would it be for physical, not magic?
Long story straight, it was not made for her, and theme similarities have nothing to do with in game mechanics. And you are by no means stopped from buying it. It will not help you at all, but you are not stopped from it either.
By the way, it looks more like Tryndamere's sword than Katrina's blades, which makes sense as his ult lets him dance on the brink of death, while he all of his skills do, and scale off of, ad. While Katrina's skills scale off of ap, and do magic damage.
Which does the Death's Dance icon look like a recolored version of?http://grabilla.com/06509-67d5d9b1-55c2-4fbc-8316-557f6caf6ed3.png
Tryndamere's Sword?
http://grabilla.com/06509-be864771-e6a5-44d0-8105-1f9f631f4432.png
Or Katrina's Blades?
http://grabilla.com/06509-9e5f9df4-9591-4a95-9ae9-2ebdaa534369.png
To me Tryndamere's Sword looks a lot closer, what with Katrina's blades not having any spikes on them at all and straight handles, while Tryndamere's Sword does have spikes, and a weird spiky handle.