He is wrong and you're wrong because it's like comparing a gnat to a flee. (And because you're not reading the post! Read the freaking post! We're not discussing point guard D in general; we're discussing it one respect: bothering bigs on pops and rolls!) One may be a little bigger than the other, but the difference is insignificant. There is a threshold that has to be met in order to really bother a big, and 6'1" is certainly NOT it.
The length is a big issue for bothering bigs on those shots. If we're talking about Horford, he takes slow set shots on the pick and pop. He doesn't jump at all. I would be surprised if his release point was higher than 7'4". Thomas on a closeout isn't going to bother that shot much while Rondo or Bradley will.
The other aspect is that while Rondo or Bradley is recovering to the point guard, those long arms are cutting off passing lanes both over the top and in the pocket. Thomas is quick, but no quicker than either of those two guards and gives up between 6" and a foot of effective space around him to the pass in comparison to longer guards.
For instance, in the pick and pops down the stretch in the fourth quarter, Teague just threw the ball over Isaiah's head a few times. He can still make that pass over a longer defender, but the angle of the pass will need to be higher, meaning it either has to be a slower pass or Horford has to catch it outside the shooting pocket and recover. Those extra tenths of a second are crucial for close-outs to bother the shot.
The importance of length to disrupt the pass is doubly important in roll situations as the only way to prevent a basket or a foul is to disrupt the catch there.