Maybe they are talking about monohulls with great big, overlapping jibs. On a lot of those boats the majority of the sail area is in the jib, so maybe in those cases the mainsail is there to help the jib, rather than the other way around. I don't think that would be true of beach cats. But maybe in light air when you are able to carry a Hooter upwind....? All theories are interesting to think about.