For posting all the anonymous reactionary bullshit that you can't post anywhere else.
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And that in turn can lead to actual antisemites being like "look at how the government lies about protestors killed, so it's no surprise they lie about the holocaust too".
Or am I reading way to much into what could just be simple propaganda meets ineptitude?
Scientists estimate that around 117 billion humans have ever lived. There are only around 8.3 billion left alive after the protests in Iran 
There are 2 dead protesters. That's the number of deaths when there's a protester death twice. So tankie, still think Iran is cool? 
I can't believe how heartless you are to a billion protestor casualties!
Iran just did 10x Tienanmen Square Genocide
I think it's just reflective of the gross and absurd way libs use atrocity propaganda but does not seem to, in this case, have any meaningful direct relevance to Holocaust denialism (unlike, for example, misrepresentations of the USSR's Great Famine in this and other styles, which directly relates to Double Genocide Theory).
I would especially point out that the sort of people who are writing this sort of shit in English are generally* going to have the stated position of Holocaust exceptionalism, which is the inverse wrong approach of downplaying the significance of non-Holocaust genocides. Not that they call themselves "Holocaust exceptionalists" of course, but that they are the type of people to call you antisemitic for comparing the genocide of the Palestinians to the genocide of Jews.
*with the important exception of Iranian diaspora Shah fans and the like, where I don't really know what they would generally say about the Holocaust.
I think the antisemitism part is being so unserious about the holocaust that you could think 1 million people were killed without leaving behind any physical evidence. Europe is covered with mass graves and other artifacts of the holocaust, because killing a lot of people is really messy in the physical world. Everyone should know this.
Nah, they have Israel now to be antisemitic about. You have to jump through way fewer hoops just to connect Israel with "The Jews"
After a blowout I had with a friend, this is just typical bigoted, racist imperialism. I was literally told, "Those people kill each other," Aka Iranian on Iranian violence. 
I don't think that's the explicit intention but they likely don't consider it an undesirable byproduct.
For contemporary propaganda purposes, its mostly just important that the number of Iranian protesters killed is larger than the number of Palestinians killed by israel.
a simpler explanation is islamophobia, look at these heartless muslims slaughtering tens/hundreds/millions of innocents
They do this all the time with any claimed statistic. I'm not really sure why. It might be a way to push it beyond a "moral event horizon" for the listener, that it gets to such a big number that you'd have to be "crazy and evil" to deny it.
It could also be a way to deflect from their own crimes. "Sure the US might have killed hundreds of children, but our enemy has killed millions so it's ok when we kill them."
I don't think it's intentionally planned that way, but the end point as you describe is probably likely.
If anything, the way the holocaust has been taught over the last century, particularly the focus on the death toll rather rather than the acts of terror themselves, means people only care about massive death tolls - its why the "deaths under communism" numbers are inflated so high - to look worse than the holocaust.