I certainly think a police officer that stumbled across such a thing would be justified in asking follow-up questions like 'hey, what's this about you going to shoot someone?' and then maybe making an arrest based on your replies/demeanor.

'Probable cause' should involve a degree of certainty, because 'possible cause' would be altogether too loose of a standard. It's possible that you're intending to shootme and you just mentioned saulpw to throw other HN users off the scent. Possibilities are only limited by the assessor's imagination.

How can you prove that it's not just me writing a dark and morbid story?

How does the cop at the traffic stop know that your username is akoboldfrying? Did you tell them? If so, why, at a traffic stop, did you do that?

Are you posting threats on hacker news while you are driving? And the cop was close enough to see your username and what you wrote? Is that why you were pulled over?