Why does the source matter?

Even if the audience is assumed to be perfect at evaluating evidence on it's merits then the source matters to the extent that the authority of the author and the authority of the presentation are considered evidence. Knowing how pieces of evidence were selected also gives information, so knowing about the can provide significant information.

And the above assumption definitely doesn't hold - people are not perfect at evaluating evidence on it's merits. Considerations about how arguments optimized through trial error for persuasiveness become rather important when all recipients have known biases and you are actively trying to reduce the damage said biases cause.

Finally, considerations about how active provocation may have an undesirable influence on the community are qualitatively different from considerations about whether a denunciation is accurate. Just because I evaluate XiXiDu's typical 'arguments' as terribly nonsensical thinking that does not mean I should be similarly dismissive of the potential damage that can be done by them, given the expressed intent and tactics. I can evaluate the threat that the quoted agenda has as significant even when I don't personally take the output of that agenda seriously at all.