I have to say, who ever the self-described "Expert" is in the link you provided has a lot of things really mixed up.
A collar tie runs from a rafter on one side of the ridge board to the opposing rafter on the other side. Are usually connected with 16d nails, are required to be minimum 2x4 lumber now but for a long time 1 x material was O.K. to use. They are usually about 1/3 the way down from the ridge board too.
Now rafter ties are used to keep the walls from spreading and ties the bottoms of the rafters, normally your ceiling joists perform this function in most homes but some homes have ceiling joists that are perpendicular to the roof rafters, hence the use of rafter ties in these situations.
Purlins run perpendicular to the rafters, usually at about the mid point of the rafters, are a minimum of the depth of the rafters and are supported by 2x4 or greater bracing on 48" centers to a bearing wall that will carry the load of part of the roof to the foundation.