It is made with the fpsc source code, so if I've read the source code agreement right it cannot be sold without violating the agreement. Unless they work out some sort of deal with TGC.
FPS CREATOR GAME ENGINE Source Code
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You will require the DBPro programming language, and upgrade 5.9 installed
in order to compile this source code and produce the FPSC-Game.exe.
You can obtain both product and upgrade from 'www.thegamecreators.com'
Used Under Public License:
1. You CANNOT make 'game creation' or 'level editor' software with this code
2. You CANNOT make a 'game engine' for the purpose of selling it as an engine
3. You CAN make a GAME with this source code for the purpose of selling it
These license terms are designed to protect the IP rights of TGC Ltd.