"As fast as C" would commonly be interpreted as "a program written in it will be as fast as a well-written C equivalent", not as "there is a C program with the same performance characteristics".
That a language is compiled to C does not mean that its compiler is going to be able to produce a C program that's as good as a that well-written C equivalent. (A relatively obvious example would be a compiler that introduces a heavy runtime, and doesn't give the C compiler enough information for it to get rid of the runtime)
It's the same with assembly code: that a compiler produces assembly does not mean the resulting program is fast.
That a language is compiled to C does not mean that its compiler is going to be able to produce a C program that's as good as a that well-written C equivalent. (A relatively obvious example would be a compiler that introduces a heavy runtime, and doesn't give the C compiler enough information for it to get rid of the runtime)
It's the same with assembly code: that a compiler produces assembly does not mean the resulting program is fast.