I think this is implying, providing the absorbed power density remains below a critical threshold, the body is able to prevent the resulting temperature rise going up to a damaging level through normal heat removing mechanisms like blood flow, sweating and radiated heat. It probably takes minutes to reach thermal equilibrium [i.e. when the body is taking the excess heat away at the same rate as the EMF is generating it] hence the six minute. It also implies that if that small heating effect causes no ill effects for 6 minutes, it’s also true for 60 or 6000 minutes.