This is absolutely a counter. There would be no assassination of an Iranian general in Baghdad had he not commanded the insurgency with the sponsored militias in Iraq.
That sounds like "another critical factor". A chain of events led to this. The parent is saying "one link in the chain is US involvement", you are saying, a different link in the chain is Iranian involvement. Both can be true, and neither invalidates the other. You've done nothing to invalidate the parent's post because you haven't removed their link from the chain, you've just added other salient links to the chain.
Stop treating blame like it's some simple, single element. It's a complicated, multi faceted chain of events.
Saying "there were other critical factors" both agrees with the parent post and doesn't counter the idea that the US was a factor.