claims the average Earth temperature during the Ice Age was 12 degrees colder than today. Is this true? How much did temperature vary during the Ice Age?
Your article says an ice age was 12F (or about 6C) colder, while other sources quote 10C colder, so there is not complete agreement on how much colder an ice age was compared to the present day. This is not particularly surprising, as there were no meteorologists making careful records in those days. Temperatures for ancient times have to be measured in very indirect and roundabout ways, for example by measuring the ratio of deuterium to hydrogen in ice cores, and there are other vaguely similar ways, all subject to error.
All that is necessary to develop an ice age is that more snow and ice should form during the winter than can be melted away during the summer. This doesn't require a dramatic fall in temperature, and if it continues for long enough you will have an ice age. After a while, of course, the increased area of snow will reflect more of the sun's heat back into space, thus increasing the extent of the ice and the amount of heat reflected away in a sort of feedback mechanism.