Earlier quoted context omitted.
What's not true? (what's 'it' that you talk about)
The article did not become propaganda.
However, that sentence "But the government of Iran probably could" made the preceding paragraphs appear to be a vehicle to deliver a meme (like a shaggy-dog story). The rest of the article could have been great, I just stopped reading.
The journalist could have made a neutral statement about what entities have the resources to crack a 768-bit key. But they or their editor chose not to.
Instead, everyone that reads the article will go away with the meme "Iran, if they wanted to, could crack 768-bit keys". Which is, by common definition, propaganda.
It might be unintentional, i.e. the journalist is riding a wave of popular opinion, which they should not do; or it might be an attempt to load the article with link bait.