Sounds a damn good hobby to me.

I don't think hobby requires building anything. Spending time actively engaged is enough. One can enjoy mathematics the way one enjoys listening to music.

On the other hand if you do want to make something, and you happen to know related math then suddenly you can use it.

For example, https://news.ycombinator.com/item?id=47112418

Building these are neither my hobby, not did I learn the relevant math for the exclusive purpose of making it. But once you acquire a few math razors you start seeing inviting fluffy yaks that were invisible before.