The item choices for Thornn may be thematic

Specifcally, in regards to the fact only one of them is an AP item.

He's a smith. He works with metal. The only AP non-hybrid item that fits the bill for that theme is Hourglass. But they had trouble with Zhonya's having too much strength before.

I think they simply wanted to include an AP option so it was more balanced even though it doesn't fit the theme. Rabadan's has been changed the least of any item in the game, so it's the best choice. Changing it doesn't lead to balance issues because it's the one item that does absolutely nothing but grant a single stat.