Funny. After googling, I've found this is actually something legitimate that people say. But, going by the wikipedia definition of "smart casual" at least, this isn't helpful. Knowing the couple involved, I think that they were worried people might show up in cocktail dresses, and just wanted to say don't do that. But, I swear, I'm missing the days back when time, date, and location told me all I needed to know about what to wear.
"I'd hate to advocate drugs, alcohol, or insanity, but they've always worked for me." Hunter S. Thompson