It's a set of C Preprocessor macros, so you don't even need to somehow process the code—you just #include the header and hack away as if it was regular C!
With C23 (nullptr, auto typing, typeof) and C11 (generics) it got more guarantees and type-related primitives. You can still do void*, but you are strongly discouraged from it.
Well, you don't have to use it all. My projects mostly use booleans, len(), max/min, and some operator aliases, because there wasn't much need for other tasty stuff yet. So give it a shot, even if for a couple of operator macros!
Indeed, FLT_EPSILON is not a one-size-fits-all solution, but it's good enough for frequent case of comparing big enough numbers, which is not covered by regular ==. So it's a convenience/correctness trade-off I'm ready to make.
Yes, code blocks in Org are executable, but I was aiming for simple embedding and zero build-time, thus conservative choice of separating README and the actual header.