yes, this is true. However, the isolation board is intended to be a thin board (about 3/8") which dissipates the heat and moisture from the quilt box in relatively reasonable manner, especially with a breeze blowing through above it.
In a modern roof, that isolation board could just as easily be replaced by a stiff aluminum screen to keep mice, etc out and provide even better ventilation.
In addition, Warre tells us that in regard to the quilt box it's intended for the beekeeper to 'stir' the contents from time to time to release moisture in the quilt material. While he doesn't stipulate, one might continue with the thinking of the minimal 2 visits/year and 'stir' the quilt and/or add/remove material at those bi-annual visits.