Well I think he was just comparing it to the corresponding erlang programme on a single core. I don't see Joe saying anywhere that it will run N times faster than a c program.
What would be dishonest would be "here's the good news for C programmers: Your C program should just run N times faster on an N core processor if written in Erlang".
To your point though, Erlang gives you scalability. Yeah, on a single core, the C program will be faster. On multiple cores, multiple processors, multiple -machines-, idiomatic Erlang should scale. C code will not. You have to tear it apart, rewrite it, figure out a distribution model, code that or find a good library, fix the bugs in your own code or work around the caveats of that library, etc etc.
Ok anyway,now that you've asked the question, maybe elaborate a bit why you think your program is slow and how do you think you can make it "go fast".