Anyway, yeah... she's a ninja, because she trained to be one during her heavily-implied thousand-plus years of life. She's trained many skills during her time alive, no doubt. After all, she had all of time to learn whatever she wanted or needed to know how to do.
Being surprised that she reveals a hidden skill like this is honestly evidence that you haven't really been paying enough attention. I'm more surprised when she reveals she's bad at something, but it's almost invariably something to do with technology, so it's at least more understandable. But she's also a fan of technology and modern conveniences, which is also unsurprising for someone that has lived most of her life before they even existed and has watched as society creates them with wonder for her people's achievements.
Her expertise is almost entirely in things that would have been vital before the Industrial Revolution, but are of less importance in modern society.