I think it's a safe assumption that the chess engine of those games was programmed by Mike Johnson. Is this important enough to ask for confirmation? Wasn't it already known that Johnson is the main programmer of the SS3? SS4 and Intelligent Chess are the same chess engine with some minor changes.

Yes there are multiple developers. The task for making the opening book, and I/O handling (pwm led outputs, LCD, multiplexed inputs with debouncing etc) could have been done by other people. Or in the case of Intelligent Chess, stuff like the graphics and the cassette interface.