Well it's a classic journalistic technique. Put two statements adjacent to each other and imply a connection but actually you can see there is no formal conjunction.
The description "from the horrific consequences of atomic bombs dropped on Japan" is written and reads as nonessential, meaning it goes without saying but is nevertheless helpful in qualifying how "scientists now knew" within context of the article's chronological prose.
I understood the parent's remark as rejecting this factual inaccuracy.
There's not much logic in my statement just an understanding of what the journalist is trying to do: dramatize something quite ordinary.
They want to establish a historical context where Hiroshima was a recent event and somehow tie this highly dramatic event to the simple fact that scientists knew radiation damaged cells. But there is no particular connection between the two since scientists already knew that prior to Hiroshima.
So what's a hack to do? Well you write one sentence. Then you write another. You don't say "because". You just stick 'em close together. That way you get your cake (drama) and eat it too (can't be called out on a falsehood).
In this case your brain does the work for them and "joins" these two statements together. At which point if you know it's false you're aggrieved - and quite rightly because it is intended as a (mild) deception.
But it passes an editor because you can just say (waving hands) "well you know it was all happening around that time - I'm just filling in the general background..."
The description "from the horrific consequences of atomic bombs dropped on Japan" is written and reads as nonessential, meaning it goes without saying but is nevertheless helpful in qualifying how "scientists now knew" within context of the article's chronological prose.
I understood the parent's remark as rejecting this factual inaccuracy.