The functions which are called by objc_msgSend do not have to be methods compiled by an obj-c compiler. You can add functions compiled by a compiler with no knowledge of or support for obj-c to an obj-c class at runtime, and then call that method via objc_msgSend.
Obviously you could come up with other ways of passing arguments to obj-c methods which would make it possible to implement your message send function in pure C, but a message send function which passes the arguments as a va_list is not objc_msgSend(), and that says nothing about whether or not objc_msgSend() could be implemented for the design they did go with in pure C.
Obviously you could come up with other ways of passing arguments to obj-c methods which would make it possible to implement your message send function in pure C, but a message send function which passes the arguments as a va_list is not objc_msgSend(), and that says nothing about whether or not objc_msgSend() could be implemented for the design they did go with in pure C.