Indeed basal metabolism takes place without much muscle movement and can therefore be considered a form of nonshvering thermogenesis,sometimes referred to as obligatory nonshvering thermogenesis.
However,in the present context,we are not referring to such processes but only to those processes which are initiated specifically in order to maintain body temperature and that can thus be called thermoregulatory nonshvering thermogenesis. this is thus the heat produced above basal metabolism,in the absence of muscle activity and with the purpose of producing heat