The introduction is quite clear that the guide uses Arduino, and using C is a possible alternative, among other options, not covered in the guide. It mentions several of the options in that same paragraph that you complain about it not mentioning.
Indeed, both are mentioned in the "programming" section, which I have either misread or skimmed over.
Edit: The options in the same paragraph are for different languages though (I saw those), but nevertheless it also mentions that they exist in general, including those for C. Well, it only mentions the official SDK for C, but hinting at more options still.
