Just because someone says that they will smack their child doesn't mean that they will do it or even have any intention on following through.
I also admit to sometimes saying to DD "if you keep on doing that I will smack you". I don't smack or have any intention of smacking her, but this is the way that I communicate to her that what she is doing is making me really angry and she needs to stop doing it now (as opposed to something that is not so serious). We usually then talk about the reasons for what she is doing and why she needs to stop. So the "threat" of smacking is not actually that, but more of a code for "this is serious", but unless you live with us and know the way we operate and the words we use on a daily basis, it would be hard to tell. I'm probably not describing it very well...