I imagine he was down modded for his weak use of sarcasm and abrasive way of making a weak point.
Gizmomagico seemed to be saying 'assange wouldn't have committed what he allegedly did; as it would retard his ability to move freely and he is too smart for that' -- which may or may not be true, and is irrelevant as evidence.
As a side note; getting offended about the content of a joke is really sort of silly. People seem to act like joking about something is somehow akin to 'doing' the thing. I don't think an argument can be made that a joke about rape makes rape any more likely; and while it may be crude and unfunny: making the claim that something is 'off limits' is really weak.
For example: 'statistically 9/10 people enjoy gang rape'