i dont think its possible to shoot 1/2" groups at 20 yards. Most arrows are 5/16" diameter, and to get the tightest group, 1 in the center with 4 around it, that means at least 3 are in a line. 5/16"x3=15/16" group. The slim carbons are about 9/32", so with those the tightest groups would be 27/32".
Unless he meant all the arrows are 1/2" apart from eachother which is very possible cause I shoot groups like that myself.