This is probably because Howard Marks popularized the idea under that name in "The Most Important Thing". He looks like FS's main source in the detail piece (https://www.fs.blog/2016/04/second-level-thinking/).

I think he does mean it as an analogy to the math concept, like ~x being first order and ~x^2 being second order for x near zero; x dominates but x^2 (the "second order") gets you closer to the truth. And similarly for x^3, x^4, ..., x^n.