"I would advise them not to commit such a strategic error."
Probably refers to the belief (mostly correct) that US Forces are overextended and there just are no troops available for a ground invasion of Iran. I don't think that's so much a threat as it is the Iranians tipping their hand; they plan (should a ground invasion occur) to either grind us to stalemate or overwhelm us with numbers (even if it means every raghead with a pitchfork must take the field).
They'll then take to the airwaves of Al-Jazeera and CNN and embarrass the Great Satan in European capitals, or perhaps repeat the Somalia debacle and drag our dead soldiers through the streets in front of MSNBC cameras.
I don't think he's alluding to nukes at all (don't get caught up on the word "strategic"), but the concentrated PR campaign which history is showing is the best way to beat a Superpower with a militarily inferior force: let the perpetually (but irrationally) guilty, nervous, and panty-bunched back in the USA win the war for you.