I have sometimes responded something like "I appreciate your concern. When you're out driving, try not to kill people riding bikes and encourage others to also try not to kill people riding bikes".
But quite frankly, it comes across to me as "I'm concerned about your personal salvation ..."
IMO opinion it's a sort vomited response when a person encounters another person living life in a way they simply can't understand. Their mouth was going to open by force of habit anyway and that was what the un-exercised brain managed to do with the words before it did.
I agree that there's really not much to bike commuting, and it takes about a year to get the hang of it. After that, it's like lather, rinse, repeat.
I regret ever having felt like I knew anything about bike commuting prior to getting rid of my car and doing it every day. A single bike commute in a year worth of driving is an honorable mention, for sure, but until you commit to riding every day regardless of weather, traffic, and what your body is doing - well, it's just to say that it's an entirely different experience. It changes you, profoundly. Add distance on top of that - plan to not relate well to the rest of the driving world at all. Including most of those who also sometimes ride bikes.