The full manual cover photo at the top of the page shows that it was published by Optimized Systems Software for Atari.

I think most Atari users interested in C or C-like programming would have, instead of a C interpreter like Tiny-C, used a) Deep Blue C <https://en.wikipedia.org/wiki/Deep_Blue_C>, or b) Action! <https://en.wikipedia.org/wiki/Action!_(programming_language)>. Both are compilers with complete library support to Atari hardware features; as I understand it, they are superior to any high-level language available for Commodore 64, and comparable to the various BASIC compilers for Apple II such as Microsoft's TASC.