Your correct Megan.The revision probably stems from the Presbyterian aversion to the Virgin Mary so the verse would have to be rewritten for acceptability by the members.
Strange however that the Church of Scotland Hymnary still shows the Apostles and Niceniene Creeds both of which mentions Jesus as "..born of the Virgin Mary".
Did Mr Knox miss these two or was he gone by the time they found there way back to prominence?
Davebhoy