In the case we are discussing, the phrase had been lifted and repeated so often that I cannot even recall the context of the original "1720 is a large number" claim. The ridicule was effective within the group which already thought little of the correspondent - but to the Lurkers following, it came across as mean-spirited and caused many otherwise useful assertions to be cast in a bad light and perhaps even, ignored.
I realize you hate the lawyerism around here but, truly, the ones we are both trying to reach are the Lurkers - who are like jurors and haven't yet made up their minds. We are not likely to convince the direct opponent. Thus when we are cautious in our presentations, we are much more persuasive.
One reason things degenerate as they do in these discussions is that all too soon there is nothing to talk about except what one side or the other refuses to understand, see, or remember. This was certainly true of the poster you mention. Intelligent dialogue is more than firing back your talking points when it's your turn. It involves hearing the other side's points and responding appropriately. Some judiciousness should be visible in selecting points for disagreement. In particular, one should not be willing to grasp at any old straw at all.
The poster you mention was also vilely personal in his attacks and was part of a group which did opposition research on and stalking of certain freepers including you and me.