So what's the catch?
Actually, there is no catch. As the authors say, if you can write C for applications targeting standard processors, you can write C for programmable hardware.
There are certainly other C-based methodologies and tools available for FPGA designs, but these tend to be focused on satisfying the requirements of the hardware design engineers. By comparison, Practical FPGA Programming in C is unique in that it addresses the needs of both hardware and software developers. While it doesn't completely tear down the fence between hardware and software engineering teams, it at least provides another gateway.
Clive "Max" Maxfield Christmas Eve, 2004