I've reverse engineered many an API, and I've never done it by decompiling the executable. Also, what's illegal about decompiling an executable?
There's merits to this claim if you're indeed implementing some advanced, niche algorithm but it definitely wouldn't apply here as all he's doing is calling HTTP APIs, a very generic and common thing to do.
One person views the "contaminated" decompiled code and writes a specification. A separate person writes the code based solely on the specification. This is an accepted method of demonstrating that there is no infringement.