I didn't write that article and I've never used that generation method, but I would assume it's usually just placed randomly. i.e. the room width and height are randomly generated within a desired range and then the position of the top left corner is also randomly picked with x in the range 1:(cellwidth-roomwidth) and y in the range 1:(cellheight-roomheight). But that article just gives a basic framework in any case - you can implement it any way you want.