I did something utterly stupid?

Well yeah it was a bit of a silly remark to make, but my issue would be why you seem to think you have a right to tell this guy to quit smoking!? I assume he is an intelligent adult that is well aware of the risks smoking poses, if he still chooses to smoke that's his choice and you have no right to tell him not to, his life, his health and his body! People that don't do something or don't agree with something always want to stick their views and opinions onto others. If you kept your beak out of his in first place business you would not have been in a position to make that flippant remark! I don't think you should mention it as you will be making a mountain from a molehill.