In the UK we had IRA terrorism, and more recently we've had Islamist terrorism. We haven't had any terrorism motivated by atheism that I'm aware of.
I'm still not clear about the point of the thread.
In simple terms, I agree with you, and it's always risky paraphrasing someone else's intent, but
@Laika can shout me down if I misrepresent...
My reading of this is that he is not talking so much about terrorism that is pro-atheist in nature, per se, but instead terrorism that is informed by atheism, or otherwise influenced by atheism.
I'm not 100% sure where I stand on that, to be honest, but there is definitely some food for thought, and using the IRA as a test case makes more sense than most.
The IRA should be primarily viewed as a cultural and political movement, specifically republican in nature, rather than a religious movement, despite the obvious links to Catholicism. But the community provided by the church was influential, and there are some really good books showing the strange impact religion played on some IRA activists (which varied greatly from parish to parish, and activist to activist). Then there are more ceremonial links, and the way Catholicism was factored into the running of meetings, etc.
Then (as you're probably aware) there was a schism. Or, more accurately, there was a series of schisms. Without drilling into the causes of these too much here (although I can if you're insterested) the IRA morphed into a Marxist based organisation, focusing on class struggle. IN that light (and I am obviously simplifying) the IRA began to identify more Ireland as a whole, and saw Britain as the enemy moreso than the Protestants. As you can imagine, this was not a smooth transition, and the Provincial IRA became the more traditional counterpoint to the Official IRA (which shrank in size, power and activity). There are other important groups to understand, particularly INLA, but let's keep it to the Official IRA and the Provos for now.
A common base actually split into two distinct guerilla organisations, with one based on Marxism and one of traditional cultural/republican lines. And this DIRECTLY impacted on the nature and aim of their actions in a very clear and measureable way.
Now, is it a mistake to conflate atheism and Marxism? Yes. Absolutely. But if you took a cultural Catholic, an atheist, who is not a Marxist and dropped them into that environment, does their atheism play a role in decisions they make? I'd suggest yes. I mean, I'd tend to think of it as their lack of Catholic belief playing a role, but...meh...
So, in short (too late!) I don't agree that atheist terrorism is really a thing, but I can see a worthwhile discussion in how atheism factors in. Or lack of belief, if you like.
My initial thoughts are that a strong idealogical cause is required for one to become a terrorist (assuming not purely mercenary reasoning) but that this cause doesn't need to be religious, and in cases where religion is involved, it's almost always A factor, rather than THE factor.
In cases where atheists are conducting terrorist activities, I'd see it as a factor removed, but that factor may in fact be a mitigator rather than an accelerant (dependant on religious belief, context, etc) so the removal of religion shouldn't be seen as a positive, but instead simply a factor.
Again, using the IRA as a reference point, the impact of religiousity on any particular activist ran the full gamut from an enabler, a non-factor, through to a reason to renounce violence.
Anyways,
@Laika can step in if I'm completely missing the point, but that's where I see a meaningful discussion here.
