Claiming otherwise is a bit dodgy. If it works the way they say, making a child class of a GPL class makes your code GPL. If that was true then if someone released a version of, say, the C runtime library that was GPL (rather than LGPL) it would make your entire system and all the code that made it up GPL'd.
The derived work, generally, is the executable not the source. The source is only a derivative work when it's a modification of someone else's source, which is almost undoubtedly not the case. The wording here is a bit ambiguous.