Yeah, a cleanroom re-write, or even "just" a copy of the API spec is something to raise as a defense during a trial (along with all other evidence), it's not a categorical exemption from the law.

Also, I find it important that here the API is really minimal (compared to the Java std lib), the real value of the library is in the internal detection logic.

This is exactly what I had in mind when I said I was simplifying :) it is a valid point.