Come on, Minimus! I'm sure that your friend believes what he's saying, but really? I imagine that there's a percentage of people who will join a religion simply because they get off on the initial love-bombing. Those people will stop and talk with the cart-witnessers and if they have nothing at all going on in their lives, they might come to a meeting and "progress" to baptism.
The congregation that I attend seems to be growing, too. Not because of the Witness given around town, but because this is a nice area and people who are already Witnesses move here, then there are babies who never learn to reason anything out, and voila! Growth!
But the carts as an actual stimulus for growth? Somebody is seeing things through rose-colored glasses.