Thanks to people that instead of refusing to debate the deniers choose to show proof of why they were wrong, nowadays, 74 years after the horrors, we have readily available proof to counter the lies of whom, with malice, try to deny it again. If people had chosen the first tactic of refusing to debate the most probable thing would be that we wouldn't have this amount of proof to counter them today.
We should always explain why something is wrong and try to convince to avoid future trouble.
The problem of course being, that Holocaust victims were asked to be more logical about genocide while denialists defended the notion that it didn't happen at all.
If someone using a tactic in bad faith means we can't use the tactic any more, we're going to be in a lot of trouble. People talking about things they don't understand and being in need of correction goes a bit deeper than "well, Holocause deniers ask questions too!".
Stallman is not a serial Holocaust denier. He is a software philosopher.
And insofar as he is an agitator, on most of his pet topics time has proven him to be right rather than to be a troll.
Holocaust deniers used this very tactic under the banner of "skepticism".
They wrote books, they gave speeches. They participated in "academic debate" as if the authenticity of the holocaust was something to be debated.
And they caused a lot of pain to those that did live through the holocaust -- only to hear from someone making false claims that it did not, in fact, happen.