Calling someone handicapped is attack against their intelligence and by proxy the validity of their argument due to their supposed stupidity.
There is no "by proxy" unless the person making the argument makes that clear themselves. If it's not specifically made as a part of the argument, then it's not a part of the argument.
That would be like saying "Oh, but Ipquarx, what you just said in that very post makes me look bad! And since that's an insult to my intelligence, you just committed an ad hominem."
In other words, a strawman.