The article makes the point that the intensity of solar radiation is far higher than that usually generated by most household heat sources.  Sol-air temps can reach up to 140 F in summer.  Radiant barriers (a white roof) can reflect much of this energy away even before it reaches a conductive insulator.  However, if you place foam directly directly in contact under a dark roof much of that solar radiation is not reflected but will conduct inward and result in high surface temperature at the foam.  Even at good R values the underside surface temperature of the foam can be still quite significant - and the energy re-radiated downward on to the ceiling.  However, the article does not consider the effect of a white roof to reflect most the energy away and then spray foam below the much cooler roof to deal with the rest of the heat energy.