It feels and looks as though they have a bit, for me, and I haven't even been doing them for very long, really.
Maybe it's a placebo thing. I don't know. But I wasn't actually expecting it to happen, is the thing. I chose them because I figured more lats and chest is always good (for me, I mean), especially in one go, and because they felt great. Seems to strengthen the intercostals, which might or might not have anything to do with it.
So yeah, tentatively, I'd say that in my limited experience so far, that "myth" does have some basis in reality. Don't know exactly how much, or how or why (not a scientist).