The C language itself does not specify directly for a heap or how it should work, but does provide pointers, etc.
malloc and its cousins are part of something called the C Standard Library, and are functions that you link to with any standard implementation of C, and those do provide access to memory that is not static or on the stack. On every platform, the way those functions actually obtain and manage that memory can be different.
C is a long-baked language and library, and now it all appears to be of a piece together. But when K&R were writing that book, that was not so obvious, and that statement is a clarification of what belongs to the syntax of the language itself (versus what is typically provided by the supporting libraries).